Unlockable Content

Unlockable Content can simply be described as extraneous content that was not available to the player from the start. The hidden content can be anything from concept art galleries, to a hidden character, even the game's Golden Ending. Methods of "unlocking" can be based on challenges, progress, time spent or sometimes just plain money.

Unlockable Content is not content that a player is required to unlock in order to complete the game. It's unrelated material. For instance, "unlocking Level 2 by completing Level 1", is just well... just playing the game (although this is, to an extent, on the way out—qv Alone in the Dark 2008, New Super Mario Bros. Wii, Guitar Hero, post-World Tour. After all, movies and music don't have to played in order).

Unlockable Content is basically the game developers showing off, and giving the player a little something extra for fun. Or forcing you to Earn Your Fun as the case may be if most of the good parts of the game are locked. Many hundreds of gamers may have played the game and not unlocked the content. Or they may have unlocked it and never used it. Sometimes a character is unlocked at the very end of the game, just to give the player some Replay Value.

Due to the increasing capacities/abilities of game systems, game developers and game engines, this sort of content is quickly becoming a Universal Trope.

A Super-Trope to Double Unlock, Post End Game Content, Secret Character, Secret Level, And Your Reward Is Clothes.

Compare 100% Completion, Golden Ending, Missing Secret, Downloadable Content.

Examples of Unlockable Content include:

Prince of Persia's Sands trilogy has a large number of unlockable artworks a few unlockable blooper videos as well, found in hidden chests throughout the game.
The Beatles Rock Band features photographs and videos that can be unlocked during the campaign, which are accompanied by trivia and history about the band, song, etc.
Soul Calibur 2: A few of the characters and a vast majority of the modified weapons are unlocked through "Weapon Master" mode.
Unreal Tournament typically has the character model for the current champion character off-limits until you beat the game.
Super Mario 64 lets you fly to the top of the castle, meet Yoshi, and gain 100 lives after you collected all 120 stars.
Super Mario Galaxy unlocks Luigi as a playable character once you've gotten all the stars in the game.
Accessing the Lost Levels in Super Mario Bros DX.
Getting to world 9 in New Super Mario Bros. Wii.
Most of the newer Sonic the Hedgehog games are especially fond of gallery mode.
- Sonic Adventure DX even went as far as to have a playable library of every single Sonic Game Gear game ever by the time all of the emblems were collected.
Most Guilty Gear games have an unlockable art gallery. Unfortunately, in order to get most of them often requires beating the majority of 'Mission Mode' which is ridiculously difficult (Poisoned Sol vs Gold Justice anyone?)
An emerging trend is to have the Harder Than Hard difficulty levels blocked until the player has beaten the game on an easier difficulty. An obvious attempt at creating Replay Value PROTOTYPE, Mass Effect, and Wet all do this, just to name a few.
Virtually every racing game made since 1995 (and some before that) feature unlockable cars. In some games there are so many, there's no way you could possibly use them all in the campaign, so it reeks of showing off. Though that's not necessarily a bad thing...
- Of special note is F-zero, which not only always(after the SNES game) has a huge number of vehicles to unlock, also has an extra-hard mode to be unlocked(dispite it already being Nintendo Hard) and usually has at least one hidden cup, and in Climax had character profiles and a storyboard of sorts that has it's own method of unlocking by a set of special-race challenges. To get everything you had to play though the GP mode and unlock all the cars through that and then play some 40-odd special races with each character, of which there are over 30.
- F-Zero GX has little interview questions and answers to unlock. For every character, for every difficulty, there are 4 questions to unlock. Four times four times fourty = too many for anyone to ever see through regular gameplay.
Contra 4 for Nintendo DS feature a lot of this.
Katamari Forever has various modes and filters that can be unlocked by meeting various requirements on each level.
Titans Quest had special codes you could purchase (or look up on the Internet...) that, when used, gave you certain armor sets, including a ninja outfit. Sadly they had level 30 requirements and definitely didn't provide appropriate bonuses for that level.
The latter Mortal Kombat games had a "Krypt" feature where you'd spend koins to unlock, in addition to characters and other gameplay features, enough bonus content to fill two DVDs.
Pretty much every Tony Hawk's game ever would have a series of secret characters, levels, videos, and cheats unlocked the second you beat the career (or classic) mode.
Super Robot Wars has this in spades Especially the Laftclanz; but since this is Super Robot Wars, it's kind of expected that you play through the game multiple times to see all the route splits and dialogue
Super Smash Bros. started out with a mandatory handful of extra characters and levels. As of Brawl, there are so many things to unlock that the game even has an extra menu to keep track of things (and show you what else to do to get more stuff unlocked). Thankfully, all of the characters can be unlocked either by playing through the Subspace Emissary mode or playing enough basic matches.
Castlevania Judgment requires you to unlock a character in story mode, then beat their story mode before you can use them in the rest of the game. If you happen to own Castlevania: Order of Ecclesia, you can circumvent this and unlock Shanoa and non-story Aeon (usually the last character unlocked) before playing a match.
Rhythm Games do this so often with extra songs, that it's easier to list non-straight examples:
- Just Dance completely averted this with its songs, and it turned out a lot of gamers didn't mind, despite reviewers thinking it was a strike against the game.
  - In the third game, you get 1 Mojo for every star you collect. With enough Mojo, you can unlock new songs, game modes and remixes. As the stars count for each player playing, having four players means getting Mojo four times as fast.
Getting all 108 characters in Suikoden Tierkreis.
Some weapons and costumes in the Dead Rising games.
- Certain rescue missions in Dead Rising: Chop Till You Drop require getting high rankings in the set of missions right before.
Akuma, Sentinel, Hsien-Ko, and Taskmaster in Marvel vs. Capcom 3: Fate of Two Worlds are unlocked in that exact order by accumulating Player Points. Additionally, there's tons of artwork and cinematics to unlock through beating Galactus with different characters.
Star Ocean: The Second Story allowed you to unlock higher difficulty settings if you collected enough voice clips. This was accomplished by playing through multiple times and recruiting different sets of the twelve available party members, then using them in battle and having them use spells/abilities/die/watch other party members die. The PSP remake Second Evolution changed that to saving at the final save point in Phynal.
In the original Advance Wars, you started with just four out of ten CO's. You then had to complete the basic campaign once per character, while completing certain specific missions (or sets of missions) with particular characters - with nobody telling you who to use when.

Are you curious as to how you get the Unlockable Content out of All The Tropes? You just have to read every single page of the Wiki. Then it's unlocked! I swear!