Pressure Plate

""Please place the Weighted Storage Cube on the 1500-Megawatt Aperture Science Heavy Duty Super-Colliding Super Button.""

- GLaDOS, Portal

Sometimes they're used for Block Puzzles. Sometimes they're to make sure you need two people to get through an area. Sometimes they just mean you don't have to press the use button. Whatever the case, a surprising number of mechanical doors are hooked up to pressure plates. Common in puzzle platformers.

Films -- Live-Action

 * In Raiders of the Lost Ark, an idol that Indy needs is set upon a pressure plate, such that removing it without putting something equally heavy in its place will collapse the cave. Unfortunately, Indy miscalculates the exact amount of weight and has to book it out of there.
 * The first of The Librarian movies with Noah Wyle features a shout out to the above. Finding the first MacGuffin, the love interest reaches for it. The Librarian pulls her back and throws a rock at it. MacGuffin moves, Enormous stone head slams down in front of the pedestal.

Literature

 * In The Fifth Elephant there's a locked room with a pressure plate near the entrance that compares the weight of people entering and leaving. So how was the Scone of Stone stolen?

Tabletop Games

 * Dungeons And Dragons players dread the activation of a pressure plate, because nine times out of ten, it will set off a nasty trap that will ruin your whole day.
 * Subverted in one of the Grimtooth's trap books for d20 systems, which features the "Click Plate", a pressure plate that gives an ominous click when you step on it... and does nothing else. No problem... unless you intersperse them in a trapped dungeon with other plates that go click just before the trap goes off...

Video Games

 * On your way to the Red Queen's domain in American McGee's Alice, you need to retrieve an insane child(who ran off the pressure plate) back to stand on it to open the gate to the portal.
 * Portal has many of these, usually involving the Companion Cube.
 * Dwarf Fortress. The key difference is that instead of pressing them yourself to do things, you get others to press them. One memorable use is to have a set of pressure plates that open and close doors or bridges, then chain up an animal on either end, load it with automated traps, and get goblins to run down it. When they hit a plate, it closes the bridge they're running towards, preventing them from pathing to the cat chained on that end, so they reverse direction and head the other way. Over the traps. Artificial Stupidity can be fun as well as !!FUN!!.
 * Various Rare games: Donkey Kong 64, Banjo-Kazooie, Banjo-Tooie...
 * There is a simple Block Puzzle in the last level of Perfect Dark which is needed to reach the final boss.
 * La-Mulana features placing bags of sand on pressure plate pedestals as a primary game mechanic. There are also the tiles which you have to push blocks onto, several of which cause other blocks to appear out of thin air. There are also the old standby "step on it and something happens" type, although they're relatively rare.
 * Let's see, is there a Zelda game that doesn't have this?
 * Zelda II the Adventure of Link, and the little-known Game and Watch game.
 * Evil Genius has them too.
 * The old Eye of the Beholder games have doors (and various other mechanisms) triggered by pressure plates in the floor. Some were activated by you, others by monsters, and yet others by items (and sometimes very specific items).
 * Ol' good Prince of Persia made extensive use of pressure plates to open distant doors; they could be held down permanently with a slain Mook or a fallen Temporary Platform.
 * The Simpsons Game parodied this among many other videogame tropes.
 * The map WAR-TankCrossing in Unreal Tournament III has pressure plates for tanks. They are necessary to take down the Cores.
 * Sometimes summoned in Eternal Darkness.
 * Lots of them in the LEGO Adaptation Games.
 * Fallout
 * Fallout 3 has traps that can be set off by a pressure plate. With a high enough Repair skill, you can disarm the plate before stepping on it- if you notice it. There's also a perk called Light Step which means that your character can jump up and down on the plate and still never set it off.
 * Earlier Fallout titles used pressure plates for a variety of traps ranging from the classic spear-thrower to explosive mines. Players could use the Trap skill to detect and defuse traps for a small amount of XP. A Critical Failure when attempting to defuse a trap usually set it off.
 * Bloodwych, which includes puzzles where you have to either leave a party member behind on a pressure plate, or (if playing multiplayer) have the other player's party stand on the pressure plate.
 * Thief: The Dark Project has pressure plates, used both for triggering traps and activating mechanisms such as doors. Weighting them down with junk can be useful for both kinds.
 * In Resident Evil 5, the Ndipaya tribe are apparently very fond of these as even their heavily-guarded ancient uins feature these.
 * First-generation Pokémon games and their remakes featured these in Victory Road, in the form of a Block Puzzle.
 * In one of Tales of Symphonia's many Block Puzzle areas, one of them (in an Absurdly Spacious Sewer) had a Pressure Plate you had to slide a block onto. There were other instances similar to this, but only the sewer one was a blatant Pressure Plate.
 * Soul Reaver beat the Christ out of this trope as part of its beating-the-Christ-out-of the Block Puzzle.
 * Dragon Age has them as trap triggers.
 * These show up in the Edutainment Game Math Blaster 2: Secret of the Lost City.
 * Golden Eye uses a puzzle like this in the Aztec level, based on The Man With the Golden Gun.
 * The Sims 3: World Adventures: There are two types of these in the tombs you traverse on your adventures: the kind you just need to step on once to (unlock a door/disable a trap), and the kind that requires constant weight (like say, from a moveable statue in the room).
 * A staple of Tomb Raider games. Many of them require a block to keep the plate pressed down so a door or other mechanism stays active.
 * A few puzzles in Gish feature this.
 * King's Quest Mask of Eternity had these. You're meant to hold them down with rocks, but you can avoid this if you manage to kill a monster so that it falls on the plate.
 * Rise of the Triad uses "touchplates" a lot; on the easy difficulty, the game tells you when you have triggered them and what they just did.
 * Jumper and Jumper Two feature several such puzzles.
 * Wii version of A Boy and His Blob has them all over the place.
 * Minecraft allows the player to craft two kinds of them: stone plates, which can only be triggered by the player or mobs, and wooden ones, which can also be triggered by dropped items.
 * Vampire: The Masquerade Bloodlines features these as trap buttons to ensnare the unwary during the endgame.
 * Patapon 3 has pressure plates in some of the dungeon levels to open gates and stuff.
 * Stealth Bastard has pressure plates as common puzzle elements which can be pushed down by platforms, player, blocks and robots.
 * Pressure plates are found throughout Legend of Grimrock, used to raise/lower gates and activate trap doors.

Web Comics
"Zola: Avoid any floorstone marked in white. It is a trap that will kill you."
 * The Order of the Stick, as a D&D-inspired comic, has this as a stapple of the traps found in dungeons by the Order. Vaarsuvius gets caught by a rather nasty one in the Draketooth pyramid in strip #843.
 * Girl Genius: In Castle Heterodyne, pressure plates are only one of the many ways intruders can get themselves messily killed.

Western Animation
"Sadlygrove: I really don't like clicks!"
 * In Avatar: The Last Airbender, the Sun Warriors' temple has a room with a ring of statues and corresponding pressure plates. When visitors perform the dragon firebending form on the pressure plates, a pedestal with a golden egg on it comes out of the ground. Don't touch the egg.
 * In the My Little Pony Friendship Is Magic episode "Read it and Weep" the protagonist of the story Rainbow Dash is reading has to steal a small stature from off of a pressure plate, referencing the famous Raiders of the Lost Ark scene. Only the protagonist in question doesn't even attempt to trick the counterweight system and grabs the statue anyway, triggering a massive Death Trap.
 * Seen in some Dungeon Crawling episodes in Wakfu. The ones from the Temple of Doom of the Dragon-Pig are especially nasty: the pressure plates change positions regularly, and the safe ones can only been spotted by looking into mirrors framing the trapped corridor. The heroes don't quite make it without triggering the trap.