PSN seems to be short on fun coop games that you can play in short, relaxing bursts. Anyone care to recommend some of their favorite coop downloadables that have active communities?

If you're talking about couch coop, then Pixeljunk Monsters is for you, but if you are talking about online coop, what the hell are you talking about? You're looking for relaxing downloadable games, playable online in small chunks, that also have really active communities? C'mon man, that's just insane.

Played fucking Pixeljunk Monsters for daaaaaaaaays. This man has taste.

Dead Nation is fun. The day it was announced I flipped my shit and went after it. So when it came out to pretty... uh, mediocre reviews, it really confused me. Me and my buddy had a blast with it. Couch co-op, online co-op. It's all there.
 
The lighting in the game is fucking phenomenal. It really is amazing.
 
I think the game is really well made and put together. The impression I got was, they took a shooter and turned into a top-down twin-stick shoot-em up. Like I said, me and my buddy had a blast. It's pretty hard though.

Dead Nation is a great game as well. You can still find games online but offline co-op is your best bet.

Hoard is really fun in multiplayer, especially with friends.

Tales from space about a blob also has an interesting co-op mode.

http://www.co-optimus.com/system/2/playstation-3.html is also a great site for co-op recommendations, you can search for games based on whether they are online co-op, same screen co-op etc.

renegade ops

@artgarcrunkle: if you're still looking for games, there's also Super Stardust HD, Rocketbirds, that Lara Croft: Guardian something, Renegade Ops, and the new Double Dragon game.

Yes, Renegade Ops is a sure winner!