I really liked Pixeljunk Monsters and Shooter. Eden is supposed to be pretty good too but I really didn't get into it. Those 2 games are hella fun though.

I also recommend Noby Noby Boy, it's something you just gotta eperience and for $5 it's well worth it.

Super Stardust HD and Soldner-X are pretty great shoot-em-up's, I haven't played all of Soldner-X but Stardust is incredible fun.

Echochrome has been pretty critically acclaimed as well, but I can't vouch for it.

and if you're into Shooters, Battlefield 1943 is pretty great although if you're really interested in it you might want to just get Bad Company 2 that just came out.