Jamestown on Steam, since it was on sale for $5 today. I've been intrigued by it since the first time I heard about the game, it's an old-school co-op bullet-hell shmup, with a 16-bit look, a story about British colonization of Mars in the 1600's, and one hell of a soundtrack.
Just look at it. Try to tell me that this game doesn't look bloody FANTASTIC.
And yes, it is just as good as it looks. There are only five levels, yes, but there are also something like five difficulty levels to play on. I may be a bullet-hell newbie, but I'm having a ton of fun with the game, and I highly recommend it, especially if it may help fund online multiplayer in the future.