VVVVVV originally came out last January, but it just recently became available on Steam and is currently 10% off (which only takes 50 cents off the five dollar price tag). I grabbed it last night and hiiiighly recommend it. It's always amazing when you find an indie game with so much thought and polish.

The soundtrack alone is easily worth five bucks, but you also get an insanely difficult (and rewarding, it never makes you replay long segments over and over) retro-styled adventure. The gameplay is simple at first but introduces new elements regularly so the challenges are always fresh. There's also a bit of exploration (with a map system) and a neat story with some clever dialogue.

I'd recommend buying the game from the official site cause I think the creator will get more money that way, but it's also a featured game on Steam, so it should be easy to find.