I'm looking for some good co-op games to play over the PC with a friend of mine, specifically games found on Steam. Our PCs aren't the most efficient things ever so we won't be able to play the newest games of this generation. Any suggestions? The games we do play together are Left 4 Dead 2 and Team Fortress 2. Trying to expand a variety of games for us to play.

System Shock 2 is now on Steam and that has co-op. Terraria is a good co-op game as well.

Both are pretty cheap can be played on weaker PCs.

If your computer can run L4D2, you should be able to run Borderlands just fine (the first one anyway, don't know about the second's specs). It's a fun game to play with someone you know, though it is a long game (easily 40-60 hours).

Not Steam but Minecraft is good fun for a few creative friends.

Yeah, we've been playing that together on Xbox, but he doesn't have it on PC like me.

Are you sure? I mean, we've already played BL and BL2 on Xbox together, but we're always wanting to experience games on the PC. Both L4D2 and TF2 run on the same engine (Source) so that is probably the reason why L4D2 would work. I'd hate to buy a game that I can't run.

Portal 2

Arma 2

Torchlight 2

Minecraft

And Zeno Clash 2 has drop-in drop-out co-op - that's a weird series.

Trine & Trine 2?

Saints Row 3 was fun when i played co op with a friend, and Far Cry 3 has some co op aswell