Randomly Generated Levels

This is when a Video Game generates a map, dungeon or level in a random (or, at the very least, unpredictable) fashion. Rather than seeing the same level designs over and over, the player gets a slightly different experience every time he plays. Roguelikes are the main users of this technique. Can lead to bad level design and bad gameplay if the maps generated are frustrating, terrible or likely to lead to other complications.

Usually the computer will impose some restrictions on this randomness, so that, for instance, there is always a path to the end of the level, and the harder monsters won't appear while the player character is at a low level. Since the designer's choices in this area often have a bigger impact on the experience than the random elements do, this trope is also called "procedurally generated levels".

There are also games that use a fixed pseudorandom seed, so the player gets to play the same world every time. This technique is used in games that contain a game world that is much larger than the memory or disk space it is supposed to fit in.

Video Games with changing seeds (The levels are randomly generated each time you play)

 * Aztec, for the Apple IIe. Each time you would get a random arrangement of preset rooms and a random collection of beasties, tribesmen and chests. Problems with the layout were eased by using dynamite to blow holes in the walls between rooms.
 * Dark Cloud and its sequel, Dark Cloud 2, were quite fond of this. Could be infuriating when it came to speed runs and spheda.
 * Chunsoft's Mystery Dungeon games, including Torneko no Daibouken, Shiren the Wanderer, Pokémon Mystery Dungeon and Chocobo's Mystery Dungeon.
 * Minesweeper randomly generates the board after the first click. Can create situations where guessing is the only viable move because there are no sentient level designers.
 * Diablo features randomized dungeon layouts which include a handful of required rooms.
 * Diablo II does the same. Now enjoy the labyrinthine jungle hell that changes layout if you try and do it in more than one sitting.
 * This was especially true before the first few patch: later on, they changed the random generator to be less annoying. Still relevant to this trope: it's still random, only in a different way.
 * Actually D2 only regenerates the levels when you play online or when you delete the map from the saved game. Playing ordinarily in single player will always keep the same map. When you're playing in open battle.net and farming, say, Mephi, it's sometimes helpful to join games to trigger the regeneration of the map until you find a configuration with the stairs to the lowest level of the durance of hate right next to the waypoint.
 * The upcoming Diablo 3 will attempt to alleviate that issue. Outdoor areas will have a more regular but still randomized layout, with landmarks such as bridges, major dungeons, and paths to the next area always in the same place. Dungeons are going to be totally random but built out of modular rooms instead of a crude assortment of walls like in Diablo 1 and 2.
 * Roguelikes, such as Nethack.
 * SimCity
 * Civilization has this trope, though you also get the option to play on an Earth-like world.
 * Even in the Earth like worlds, both the resources and the nation starting positions are randomized, leading to some very weird situations, such as being the Aztecs in Spain sailing off to conquer the Spanish in Mexico.
 * Toejam and Earl
 * The first two Dragon Quest Monsters games. In the sequel, there were predefined "main" worlds and the sub-worlds were randomly generated; in the original, all worlds were randomly generated.
 * In Pikmin 2, the caves are created this way.
 * Infinite Dungeons official module for Neverwinter Nights.
 * Battle for Wesnoth allows for randomly generated maps. Also, two of it's campaigns have randomly generated maps (one in each). Random cave maps, however, tend to favor chaotic units far more than lawful units, as the cave generator doesn't make any lighted spaces yet.
 * The Command & Conquer series allows for randomly generated multiplayer maps.
 * F-Zero X has a mode where you play in randomly generated tracks, sometimes with sadistic results.
 * X-COM has levels randomly generated based on pre-existing bulks of tiles (for instance, buildings) - with the exception of your own base.
 * UFO Alien Invasion does much the same.
 * Age of Empires took it to the extreme by letting people script the terrain generation.
 * And Arena. To anyone who's played both of those, the static dungeon maps in Morrowind and Oblivion still look like they started with the same random generator, and have the same architectural (or subterranean) plausibility.
 * Hellgate:London has this as one of its features - not surprising, since many of the developers came from the team that made Diablo.
 * Spore - Initially planets are generated and randomly populated with creatures, then an entire galaxy of planets and space civilizations.
 * Dwarf Fortress randomly generates every starting world. This being Dwarf Fortress, it also generates the history and legends of the land and tends towards realistic landscapes. Nothing like the taste of geology in the morning!
 * Lufia: The Legend Returns and The Ruins of Lore both have all their dungeons randomly generated. Lufia II did this before them, but just in the Ancient Cavern, an optional portion of the game.
 * Persona 3 randomly generates most of the non-boss floors of Tartarus, although it follows a few design rules. For instance, floors between a Tartarus Boss and a plot-determined barrier will invariably be smaller than the norm, and the party will usually appear extremely close to the stairs to the next floor (with the small inconvenience that The Reaper will spawn that much faster, too.)
 * Persona 4 does the same thing with its non-boss floors, though there are one or two premade floors without bosses on them in the game.
 * The Worms series allows players to battle on randomly generated maps.
 * The Bonus Dungeons in Nippon Ichi games are like this.
 * "The Dark World" (Hell) in La Pucelle.
 * Item World in the Disgaea series. (and Class World in Disgaea 3: Absence of Justice).
 * Dungeonman Dungeons in Phantom Brave.
 * Some regular stages in Makai Kingdom have randomly generated portions. Dungeons created through the Sacred Tome are always randomized.
 * Z.H.P. Undying Ranger vs Darkdeath Evilman is Nippon Ichi's attempt at a Roguelike and thus completely based on the trope.
 * In SWAT 4, the layout is always the same while the people in the buildings are placed differently every time.
 * Spelunky is a rare Platform Game example.
 * COMPUTE!'s Third Book of Atari includes the Atari BASIC source code of a game called Castle Quest, in which each room has random placement of walls and enemies.
 * Rise of the Triad bundles a program with the registered version called RANDROTT for generating random levels. Pick different parameters and a different seed value for a different set of levels. Quite a few user-made levels available online are actually modified RANDROTT levels.
 * Every level and world of Infinite Mario Bros.
 * In Minecraft, by the same developer as Infinite Mario Bros, this trope is taken to ridiculous levels: the random level generator is used to produce levels larger than the entire surface area of Earth.
 * Water Warfare randomly generates all its maps, though in each of the four map themes (Playground, Beach, Plaza, and Nature Park), certain features will always be consistent. Certain templates will also show up with reasonable consistency based on map size (small or large), game type (Battle Royale, Deathmatch, Treasure Chest, Checkpoint, Point Rally, Defender), and number of players (2-8). For example, a 1-on-1 Battle Royale on the Playground will always have two hills and no underground tunnel, but adding just one more player allows a tunnel to appear. But no matter how many players are added to a Treasure Chest battle, if it's on the Playground, there will never be a tunnel. (In Mission Mode, however, the maps are always the same for each mission, and not random at all).
 * Yoda Stories.
 * The Microprose game based off Magic: The Gathering has a randomly generated map. Enemies are encountered randomly. Dungeons, where you get random cards, are generated randomly and appear in random locations. Enemies attack towns randomly; if you did something for that town to give you an extra hit point, you lose it if you don't stop them; if one wizard (There are five, one for each color of magic.) takes over three towns (four if you have a special item), game over.
 * The dungeons in Recettear are generated this way.
 * The original Castle Wolfenstein has pregenerated rooms whose layouts didn't change; however, the order and connection between rooms was randomized at the start of each game.
 * RuneScape has the dungeoneering skill, which is firmly based on this trope.
 * The Inazuma Eleven series has training centers. The course consists of a rectangular grid of rooms with doors between them. You start at a random room in the bottom row, one (possibly more, but you're locked in as soon as you find one) random top-row room is designated as the goal and is larger than usual, and the two are always connected by a path through a series of unlockable doors. Aside from the goal room, each room also contains either a free item or a battle at the center, and picking up the item or winning the battle opens all unlockable doors in the room (from the side you're on only; doors can be unlocked twice, once from each side). Losing a battle kicks you out, while reaching the goal room pits you in a full-fledged soccer match which gives everyone on your team a permanent stat boost if you win. Each time you play, the set of unlockable doors and each room's contents are randomized. As you progress in the main story, the grid gets bigger, the battles get harder, the item drops get better, and the stat boosts for clearing the whole course are also increased. Additionally, the layout tends to vary based on the course you pick (which determines which stat(s) get boosted if you win); the Stamina Course in particular tends to have the path snake around and go through nearly every room in the grid, with branches leading to dead ends being few and short, while some others tend to have maze-like layouts.
 * Tales of Symphonia has Niflheim, while its sequel has Gladsheim. Both consist of nodes on a rectangular grid with connections between them. Niflheim is a timed 15-level dungeon with no save points, and each level requires you to either defeat all enemies on it or find the exit. Gladsheim is an untimed 10-level dungeon with a save point on the 3rd and 6th level next to the entrance, and the 10th level has a boss fight and nothing else. Apart from the 10th level, each level is a 8-by-8 grid; you start on a random non-edge square and have to go to all 4 corners and defeat the enemy at each corner.
 * Terraria can randomly generate maps several times larger than the application itself.
 * Get Medieval (a Gauntlet (1985 video game) clone) had a game mode that would randomly generate dungeons.
 * Canabalt
 * Robot Unicorn Attack
 * Oasis has randomly generated levels based on the difficulty, campaign and level.
 * In Space Panic, the ladders connecting the platforms were randomly placed.
 * Covert Action break-in maps has each location different in every game. This also leads to creation of many bizarre places - rooms accessible only via jumping over furniture, a single room with separate entrance that belongs to the same organization, but is walled off the rest of the building, etc.

Video Games with fixed seeds (The developers randomly generate a level, and put that level on all copies of the game)

 * Elite has eight galaxies with 256 procedurally generated planets each (2,048 in total). The seed for generating a galaxy is 48-bit, and the game was initially going to include a galaxy for every possible seed value (282 trillion galaxies with 72058 trillion planets in total), but they thought that would be overkill.
 * Mind you they had problems with the mere 2000 they chose. First they had to rewrite the seed several times when it generated planets with obscene names and second there are a few systems that, if you arrive at (Via the hugely expensive, one use, intergalactic hyperdrive), you can never leave because they are too far from other systems.
 * Elite 2: Frontier fits a galaxy of 100 billion star systems on a 3.5" DD floppy. All of this is procedurally generated apart from a small scripted islet which contains the planetary systems of Sol and many other familiar stars.
 * Infinity: the Quest for Earth uses procedural generation to create billions (literally) of different star systems (including stars, planets, moons, and so on). Since ths seed is always identical, however, every player sees the same universe.
 * Daggerfall has, with a few hand-crafted exceptions, over 15,000 procedurally generated towns, cities, villages and dungeons. In this case they were generated on the developer's computers, then placed on the disk.
 * Oblivion also did this with parts of the field outside dungeons and towns.
 * Starflight may have a scripted overworld (although the star systems feel quite random-generated), but at least the planetary maps are generated.
 * The Sentinel has 10,000 unique levels (or "landscapes") and still fits in the 48K RAM of ZX Spectrum.
 * It may be even better than that: The original version of The Sentinel was written for BBC Micro. Depending on the model the amount of RAM could be 16, 32, 64 or 128 kilobytes. So, if the BBC version ran also on Model B (which had 32 kilobytes), it could mean that the versions for C64 (done by the creator of the original BBC version himself) and ZX Spectrum (done by Mike Follin) don't even come that close to using the entire RAM of their respective platforms.
 * The Explorer, an obscure 8-bit computer game, is the extreme example in the 8-bit world with its 4 billion unique locations.
 * Noctis is a space-simulation game which generates a galaxy with a radius of 90 thousand light-years. The entire purpose of the game is explore it and upload findings into an online guide so that others might find them.
 * Sonic 3 and Sonic & Knuckles have bonus levels that have you running around touching all the blue spheres (which then turn red) and avoiding red ones. Lock-on Sonic & Knuckles with any non-main-Sonic game made before it, and you'll get a Sonic 3 style bonus level unique to that game, and other copies of the same game will play the same stage. The idea is that gamers will want to check out their whole library of games for bonus stages, but this was an unadvertised feature, possibly because it didn't work with games made after Sonic & Knuckles. There's a lot of different bonus levels to find, but there's even more if you connect S & K to the original Sonic The Hedgehog, which has more than 134 million levels (don't get too excited though, repeat levels start appearing after a mere 120 million), and lets you revisit ones you've already played through passwords.
 * The .hack games have dungeons created by selecting a group of keywords, which are generated from those keywords, except for a few plot-critical dungeons.
 * The grottoes in Dragon Quest IX
 * Soldier of Fortune II's Random Mission Generator generates maps from pseudo-random fixed seeds that can be chosen by the player.

Webcomics

 * In A Modest Destiny, Fluffy built such a dungeon.
 * Speak With Monsters illustrates a potential problem with this, as applied to Tabletop RPGs, here.

Tabletop Games

 * Many table top rpgs include systems of randomly generating dungeons. Likewise, there are many third-party programs designed to generate random dungeons for tabletops. (One for D&D 3.5, for example: http://donjon.bin.sh/d20/dungeon/ )
 * One of the two realities in the game The Splinter is an infinite, randomly generated fantasy dungeon. The rulebook contains plenty of random dice tables to generate a tiny fraction of this infinite dungeon for the players to explore.
 * "The Invention of Randomly Generated Dungeons" from Playing at the World describes some of the earliest approaches to random dungeons by Midgard and D&D developers.