Definitely Critter Crunch and PixelJunk Monsters.
Critter Crunch is just a great game all around, and takes great advantage of the unique capabilities of the PS3 hardware: 1080p graphics and lossless surround sound audio. It also has a great variety of gameplay, between the levels in the story mode, the puzzle mode, challenges, and survival. If you ever get bored or frustrated with one aspect, just switch. It has both local and online co-op and versus modes, but even in the story mode two heads are better than one. You also can't beat the price: $6.99!
PixelJunk Monsters isn't as visually or aurally stunning, but is great co-op fun nonetheless. Right after my husband and I finished PixelJunk Eden (and Encore), we got Monsters. We finally beat Monsters and are now in the process of beating Encore. We're hoping they eventually bring the new levels from PixelJunk Monsters Deluxe to the PS3 with another add-on pack.
PixelJunk Eden is quite amazing, but is best played in co-op mode with another gamer. While Eden is amazing, and a more innovative title than Monsters, Monsters is more friendly to casual players.