Selective Gravity

"Q: APPLES DO NOT FALL UP A: They're more like giant cherries..."

- The official FAQ for I Wanna Be the Guy

Could be considered the video game version of Gravity Is a Harsh Mistress.

Simply put, in just about every video game ever created, most notably platformers, gravity is applied only to some things whilst everything else is completely immune to it.

While gravity in a game can clearly exist, you and your enemies being susceptible to it, you will find that special objects, power-ups, rows of blocks, coins, entire platforms and even some enemies can simply levitate motionless in the air with no wings or otherwise believable means of overcoming gravity's supposed pull. This is especially prevalent in space levels. Even objects that are on the ground, lie in a position where they should fall in Real Life, for an example, coins being upright.

You, on the other hand, can rest assured that you will always plunge to your death if you step off the edge of a cliff. The player is always gravity's bitch. (Except if you happen to be playing Boulder Dash; then it works the other way around entirely.)

Comics

 * In Don Rosa's Scrooge McDuck story "A Matter of Some Gravity", Magica De Spell uses a magic wand to change gravity for Scrooge and Donald Duck, causing them to fall sideways, and later upside down.
 * Taken to extremes in a very old X-men comic. Scarlet Witch was able to levitate by altering the probability that gravity would effect her.
 * During Blackest Night the living planet Mogo increased his mass in order to pull all organic matter closer to him. Just organic matter.
 * Take note that Mogo is a Green Lantern, and thusly capable of screwing with natural forces quite a bit.

Film

 * In Ultraviolet, Violet possesses a device that adjusts gravity based on her current orientation. Not only does she use it to attack agents of the Arch-Ministry from ceilings, she also falls towards a glass ceiling and break through it to land right side up on the roof of the building, she also uses it on her motorcycle in a Crowning Moment of Awesome to try and escape pursuit horizontally through the city.

Video Games

 * Kirby controls his density, ranging from slightly heavier than air (so he can stay airborne by flapping his flippers) to granite-like (when he uses the Stone power).
 * In Sonic & Knuckles's Death Egg levels, there are switches that reverse gravity and let the player walk on the ceiling; yet they only affect the player and one boss, not any other enemies.
 * Sonic Adventure 2 has the infamous Mad Space stage where three of the floating land masses have a gravitational field, with meteors orbiting them and all. The controls on the land masses however...
 * Sonic 2 and Sonic 3 & Knuckles have a bizarre, possibly unintentional variation: if you go to a place where the screen loops vertically and there aren't any slopes or obstructions, it's possible to keep falling endlessly, and you keep constantly accelerating as you do - but because of the way falling is coded, if you fall too fast, you get integer overflow and start rocketing upwards, decelerating rather than accelerating, until you stop completely and it starts all over again.
 * Delicious fruit OF DEATH that falls up or sideways in I Wanna Be the Guy: The Movie: The Game.
 * Everyone also forgets that Spikes can fly in every direction as well. Including ones that it isn't pointing at.
 * Oblivion contains one of the most startling examples: paintbrushes can be picked up, placed into and removed from containers, and so on, but aren't affected by physics. A dropped paintbrush will remain hovering in midair wherever you placed it. This bug leads to some entertaining possibilities for the canny player, such as building stairways to otherwise inaccessible locations and setting up sniper's nests where melee attackers can't reach you.
 * All items work this way in Morrowind. Simply stack them on top of each other, then remove the support to leave them hovering in midair.
 * The paintbrushes are particularly dangerous as they are cheap and can be used to sequence break the whole plot.
 * Interestingly, this paintbrush bug unintentionally makes up for the fact that the player-levitation spell was not carried over from Morrowind. Naturally, both the spell removal and the inclusion of physics effects for all objects were intended specifically to avert different aspects of this trope.
 * In Elastomania, not only do apples typically float in mid-air, but some of them change the direction of gravity when you collect them.
 * In Metroid Fusion, you fight a boss (appropriately) named Nightmare who changes gravity on you partway through the fight, changing your jump and movement speed and effectively rendering your missiles useless.
 * Unless you get really, really close.
 * This boss returns (or rather first appears) in Metroid: Other M. Leading up to it are areas where gravity is flipped upside down or increased. Samus' gravity suit nullifies these effects.
 * In the platforming game N, the player character is pretty much the only element in the game that gravity applies to - the enemies, terminals, and even the gold all float regardless of gravity.
 * Can be partially explained by having stuff mounted on the wall, since the game is strictly 2D.
 * Averted most of the time in LittleBigPlanet: All collectibles (Score and Prize Bubbles, and Player Enhancements) obey the physics engine (though a level creator can simulate this trope by attaching String to a bubble; used frequently), as well as nearly all Materials, Decorations, Gameplay Elements, and of course Sackboy... except for one single material: Dark Matter. No, not the stuff science tells you about. This kind of Dark Matter is easily visible, fully touchable, and floats. Not just regular old floating (you're thinking Floaty Materials there), Dark Matter is literally anchored in place. It just doesn't move. It can't be destroyed, it won't move if attached to something (though things attached to it do move if it's like a Piston or something as the connector), and no amount of force can change its position. The only thing that can move it? Sackboy's Popit Cursor (and only in Create Mode). Before patch 1.07 was released, it was possible to change the texture of a material without changing its properties (though the glitch is now fixed). Grabbable Wood, a piece of Dark Matter that responded to a Motor Bolt, you name it. This video of a level appropriately named "What's the Matter?" takes the glitch to its fullest. Now, the only way to make regular Materials float this way is to glue them to Dark Matter. Still works, but not as cool-looking.
 * In Irem's classic NES game Metal Storm, your Humongous Mecha possesses the power of Gravity Shifting, which enables it to switch gravitic orientation between right-side up, upside down and vice versa at will. This is an important mechanic to get through many levels of the game, and one power up enables you to use Gravity Shifting as a charging fireball attack.
 * Tetris pieces descend into the well and lock into place once they've remained motionless for a half second. A row of the well full of blocks empties out, and then the blocks above it move down by exactly one row. It turns out that there are ways to clear lines and leave blocks completely unsupported. Only a few games have "recursive" or "cascade" modes that allow groups of such blocks to fall into gaps below them, which allows the sort of chain reaction seen in other games like Dr. Mario and Puyo Puyo.
 * Gravity Man's stage in Mega Man V for NES has barriers that change the direction of gravity when Mega Man crosses them, affecting Mega Man and some enemies but not others.
 * The Outer Wall from Cave Story. The player character falls down like normal, but the hopping enemies and any powerups affected by gravity fall sideways, against the wall.
 * Happens in Backyard Basketball. But with a cheat.
 * In Jazz Jackrabbit 2, powerups usually float, but if you hit them with your weapons, they will start to be affected by gravity.
 * The Incredible Machine series is made of this trope. Every item is basically either affected as normal by gravity, or floats immobile in the air. Whether an item floats or sinks is based far more on gameplay considerations than logic: for instance, paint buckets, cats, baseballs, and candles fall; baskets, alligators, 8-balls and lava lamps don't.
 * 8-balls obey gravity, it just presses them agains your monitor's surface... The rest of floating things is supposed to be attached to the monitor's surface, apparently. Just imagine a wall-mounted alligator.
 * Similar to the Super Mario Galaxy example above, Final Fantasy XIII has Cocoon, a giant rock that sits in one place (while being inside the atmosphere of Pulse, the main world) with a civilization on the hollowed out inner walls. The source of light and heat comes from a fal'Cie floating in the center, though there's also big chunk of the shell that was blown off during a war. (Information can be seen here) Somehow, water is visibly falling from Cocoon on to Pulse. Considering this is Final Fantasy, it's best to say a fal-Cie did it.
 * In Minecraft, only certain types of blocks (sand, gravel; partially, fluid blocks) are affected by gravity. Other block types are not, allowing them to float in midair.
 * Sometimes the blocks that aren't supposed to float might still do so if they were generated that way. Such floating blocks typically fall as soon as you do something affecting them, though.
 * Clonks, animals and objects fall, buildings and the ground itself float happily. Oddly, Floating Platforms can still fall apart due to earthquakes.
 * In EverQuest all computer controlled characters are immune to falling damage. This is to prevent players from exploiting it to kill monsters. Casting levitation on yourself and leading a monster chasing you off a cliff would be too easy.
 * In World of Warcraft, some game objects, like ore deposits and treasure chests, have collision properties (you can stand on them), but despawn after being consumed or after a period of time. This leaves a character who was standing on them apparently hovering in midair until they make any movement, at which point gravity notices them and they fall normally. Similarly, in older versions of the game, corpses became instantly immobile upon death. If a player died while airborne, their corpse (and later skeleton) would hover at that exact spot in calm defiance of gravity. This can still be observed under certain circumstances.
 * Flavour text from Heroic Armies Marching reveals that the Daevas are immune to gravity because of their magic.
 * Rick Dangerous has a variation. In certain areas, if you place a bomb next to a movable block, then the explosion will cause the block to fly away towards the bomb.
 * Amusingly omnipresent in Subnautica -- you, your dropped possessions, and the occasional submarine are the only things that will reliably fall when unsupported. It is possible to construct an entire base in mid-air, with no support whatsoever, and it will just hang there.  No, scratch that -- it is possible to create a network of corridors and rooms spanning the entire playable area of the game in mid-air, with no support whatsoever.

Web Original
"Dr. Wood: It has come to the attention of the higher level personnel that someone has been stealing SCP-283 for use as a paperweight. I don't care how 'zen' it looks, any further misdemeanors will be dealt with severely."
 * Straight from the SCP Foundation, we have SCP-283. It is essentially an ordinary rock in every regard, apart from the simple fact that it falls sideways, all the time.

Webcomics

 * Xkcd plays with this here.
 * Believe it or not, deconstructed in Kid Radd. As in videogames, gravity is not a function of the world, but of the individual object.

Other

 * Fantasy artist Robin Wood's "Theory of Cat Gravity" posits that cats use Selective Gravity as a means to assert control over their owners. When a cat is happily lying on top of you, it increases the localized effects of gravity, making it physically impossible for you to dislodge the animal until it chooses to depart.