Local co-op I recommend Human Fall Flat, best fun I’ve had with someone in quite a while. Will follow for other suggestions though Or Portal 2
dead rising 2
Killing Floor 2, Miscreated, The Forest, Dead Effect 2, and some others I can’t think of cuz I’m at work. But miscreated sounds not good for any people. Sucks that when only have 7gb of ram.
Dead rising and Portal. Looking for something more in the realm of adventure/exploration. I mean to say something that’s not like “mount your friends” or “smash bros” if that makes sense.
Brahalla is pretty sweet. And it’s free. It’s like SSB
resident evil 5
I know it’s not steam but Diablo 3 was a very fun co op game.
Tera was a fun, free mmo
Kane and lynch.
For online co-op, Borderlands 2 is worth a try. Can’t comment on its offline co-op though; I dunno if it has it. Yeah there’s LAN and you can get a tool so you can play up to 4 players split-screen 🙂 It also works with Borderlands the Pre-Sequel.
If you are looking for local co-op, and like zombie games check out How to Survive and How to Survive 2. Also there is always Left 4 Dead 2 if you don’t mind learning a couple console commands to get it set up