VVVVVV is a retro styled 2D platformer by Terry Cavanagh, creator of dozens of free games. You play as the fearless leader of a team of dimension exploring scientists who inadvertently crash, and are separated. The game involves exploring the strange world you've found yourself in, and reuniting your friends. VVVVVV explores one simple game mechanic: you cannot jump - instead, you reverse your own gravity at the press of a button. The game focuses on playing with this mechanic in a variety of interesting ways. The game is designed not to artificially gate your progress. In VVVVVV there are no locks, no power-ups, no switches, nothing to stop you progressing except the challenges themselves.
Retro-graphics vis a vis Commodore 64, an epic soundtrack, and extremely well-designed levels. Absolutely superb. I have played this game through at least 10 times. I have the soundtrack, because those tunes kept getting stuck in my head. Ridiculously fun, but at the same time, particularly rage inducing.