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.
The first time I played this game, I fell in love with it. The fantastic music and the awesome style of the game are just two of the major reasons I love it so much. The gameplay is fast paced, difficult, and a ton of fun. I have yet to try more custom levels, but from what little I've played around with the level editor it's easy to use and functional. I very highly recommend this game and it is very easily worth the $5 you would be spending on it.