Cave Story is the best freaking game ever. It's this 2D side scroller adventure game in which you're this guy who's trying to help out these rabbit-like people. There's more to it than that, but Cave Story is really one of those games where you have to play it for yourself to truly know what it's all about.

Plus, it's free (as in beer), so bonus. And it's got Mac & PC versions, so double bonus.

You can find everything relevant at this site .