There actually are too many good ones to choose from...
Some personal favorites:
Flower
Pixel Junk Eden (Just getting into this, really awesome gameplay. Plays like an old atari classic that somehow never was (but with cooler physics.)
Noby Noby Boy (It really is worth a try for five bucks, a new patch added tons of music and four player multiplayer for free. Plus, they just made it to MARS!)
Comet Crash (totally unadvertised, really inventive and solid tower defense game where you can also release your monsters, and your opponents have their own towers. Also, you all fly spaceships around. Got an 86 average on metacritic.)
Crash Commando -- I actually really enjoyed this one while I played it. If you like contra, this is basically a contra deathmatch game with online play for 16 players. There are a ton of weapons, powerups, and vehilces.
And of course Warhawk belongs on here, if it really counts as a DL title...
