Space Zone

Space, as presented in video games, comes in two varieties:


 * The Shoot Em Up version of space, in which you can only go in one direction, possibly thanks to Two-D Space -- which by a crazy coincidence happens to be the direction swarming with enemies.
 * The platformer version of space -- which is basically like every other platformer stage, except that you might jump a little higher due to lessened gravity. It may take place in a space station, densely-packed asteroids or on an planet or moon surface where the atmosphere is missing.

Needless to say, Space Does Not Work That Way.

Shoot Em Ups

 * The Lylat System of Star Fox, and former Trope Namer.
 * Gradius, R-Type, and all their variants and spinoffs. Special mention to the Genesis title Whip Rush. Several levels feature segments where your titular starship flies up, down, and even backwards, all while obediently facing and shooting to the right. It requires getting either missile or flamethrower powerups to be able to shoot in the direction you're going.

Platformers

 * Super Mario
 * The entire basis of Super Mario Galaxy. Space Junk Galaxy is probably the most straight version of this out of the main fifteen levels.
 * The Space Zone in Super Mario Land 2 Six Golden Coins is the Trope Namer. Minor subversion here in that Mario does have to wear a spacesuit, and the gravity is lessened.
 * Also shows up in Super Paper Mario. You do need a space suit, but the space suit is merely a fishbowl on your head. One Boss Battle does an Unexpected Gameplay Change and turns it into Shoot Em Up space. Hop on Carrie and it's already a Shmup in space.
 * In Paper Mario the Thousand Year Door you go to the moon as well, without any oxygen gear. Goombella tells you not to think about it too hard.
 * Star Man's stage in Mega Man 5.
 * The seventh chapter of Disgaea 3 takes place on a path that goes through outer space, and connects the game's netherworld with the human world (Which exist as separate planets, contrary to what the names may imply).
 * Sonic the Hedgehog series. The space station levels are mostly platformer levels.
 * The Death Egg Zone from Sonic & Knuckles. There's also a shoot-em-up space zone: the Doomsday Zone.
 * Starlight Carnival and Asteroid Coaster Zones in Sonic Colors.
 * Sonic Adventure 2 has a bunch of these: Meteor Herd, Mad Space, Cosmic Wall, Final Rush, and Final Chase all come to mind. All of the other levels set on the ARK are lesser examples.
 * X Zone and Moon Zone in Sonic Advance Trilogy
 * Egg Utopia and True area 53 on Sonic Advance 2
 * E.G.G. Station in Sonic the Hedgehog 4
 * Many Kirby levels, often featuring the especially delicious UFO enemies.
 * "Project F" from Superfrog is a shoot-em-up space level, but World 6 after that is set on a space station in platformer space.
 * Rocket Knight Adventures, a platformer for Sega Genesis, had a shoot-em-up level in orbit towards the end of the game.
 * The Moon in Duck Tales. "Hey Uncle Scrooge, you need a suit up there! How are you alive? You need heat! Also air!"
 * The space levels in Ratchet and Clank (at least in the first game) seem to avert this trope. Ratchet can't go out there himself, but has to send his Robot Buddy Clank to explore. Then it turns out that the only thing Ratchet needed was an oxygen mask, and suddenly the space levels function exactly as the planet levels. Gravity works the same, there's no need for a pressurized suit, and even propellers work just as normal! Later games gave Ratchet some sort of commando suit, which kind of made more sense.
 * "Space A Go-Go" from DK: Jungle Climber.
 * Mass Effect 2, when navigating in star system or cluster. All star systems and planets, plus ships and stations are in same plane.
 * Although the game is careful to point out that this is a feature of the Galaxy Map and not an actual reflection of what the galaxy looks like - its the same reason that the Normandy is shown bigger than whole planets and how you can fly it through suns.