Co-op game recommendations anyone?

My boyfriend and I need a new co-op game to play, so I was wondering if I could get some recommendations.

I'd prefer games we can get on Steam or online elsewhere, as well as things under $20 if possible.

Here are other games we've played that we really like: Team Fortress 2, Left 4 Dead 1 and 2, Borderlands, Alien Swarm, Dota 2, Killing Floor, Terraria, Counter-Strike: Source, Portal 2, Minecraft, various SourceMods

Also I've been considering getting Bunch of Heroes or Shoot Many Robots. If anyone here has played them and wants to offer advice on whether to get them or not, that would be awesome too.

Any help will be much appreciated! Thanks!