A platform game (or platformer) is a video game characterised by requiring the player to jump to and from suspended platforms or over obstacles (jumping puzzles). It must be possible to control these jumps and to fall from platforms or miss jumps.
Platform games were, at one point, the most popular genre of video game, and still one of my favourites!
For a game developed by a single guy you can’t help but be amazed by the quality of work that went into it.
The main character awakens in an underground cavern with no memory of how he came to be there. He finds a village populated by rabbit-like creatures called Mimigas, who are being persecuted by the Doctor. Two of the Doctor’s servants, Misery and Balrog, are looking for a Mimiga named Sue, but mistakenly abduct a Mimiga named Toroko. The player’s main quest is to rescue Sue, save the Mimigas from the Doctor, and escape the floating island on which the game is set. There are three different endings: a good ending (naturally the hardest to get, involving a very difficult final stage and two extra bosses, including a multi-phase final boss), a normal ending, and a bad ending (which occurs about halfway through the game and requires little effort to get). The endings are based on the player finding various secret items.
In Spelunky you are an Indiana Jones like explorer exploring a system of caves for reasons unknown.
Spelunky is a cave exploration / treasure-hunting game inspired by classic platform games and roguelikes, where the goal is to grab as much treasure from the cave as possible. Every time you play the cave’s layout will be different. Use your wits, your reflexes, and the items available to you to survive and go ever deeper!
Iji is an action-packed strategic platform shooter with a detailed story, large levels with multiple paths, powerful bosses and lots of secrets. This game can be described as a platformer mixed with RPG elements and awesome weapons.
Knytt Stories is a very lo-fi atmospheric game with each level being a little adventure.
Knytt Stories, features Juni, a character who shows up commonly in Knytt myths and stories. In the game, the player acts out these stories (also called levels or worlds) as Juni. The two stories that comes bundled with the game, “Tutorial – Learn How to Play Knytt Stories” and “The Machine – A Save-the-World Adventure,” are fairly short, but the game comes with a full level editor and support for many extra levels in game, loaded from .knytt.bin files.
Bonus (27/05/10): Nifflas has released FiNCK
Not a platform game in the strict sense but it does feature some awesome 2D fighting. OpenLiero is a hack of the original Liero that enables sound again. There is also an update version with multiplayer functionality called OpenLieroX.