There's quite a few downloadable gems these days. If you're into mega man, 9 & 10 are fantastic retro-style games. You can pick up the whole Pixeljunk pack right now for $15 as part of a sale (Racer/Monsters/Eden as a bundle, Shooter separate).
Not going to repeat everybody else's lists other than to reiterate that Demon's Souls is definitely a 'must-have' if you're into a brutally difficult dungeon-crawler.