For Steam, VVVVVV is a bit like Super Meat Boy (in terms of retro-ness and difficulty) and Braid is also on Steam. For XBLA, Alien Hominid and Castle Crashers are both good, or so I've heard. Castle Crashers is also on PSN, and Scott Pilgrim Vs. The World is on both as well. For Wii games, well...can't really help.