I've been playing The Binding of Isaac by Edmund McMillen of Super Meat Boy fame a lot lately. The gameplay is a mix of Smash TV style twin stick arena shooter and the multi-room dungeons from the original Legend of Zelda. The content is randomized, games are quick and you will die a lot. There is also a surprising amount of depth from the various random item drops that change your character. The story is a cheerful mix of deranged biblical occultism and (often literal) dead baby humor.