Dead Space 3 is also soon going to have co-op and it's already got effectively co-op VS with teams of 4 vs 4 in Dead Space 2 and those games always run great on PC even if the PC is below recommended settings (seriously, it's some of the best PC optimization I've ever seen, aimed at budget gamers).
Also don't know your console, so I'll just name a few and their systems.
PC: Portal 2 (new-ish, but if you can run it then try it) Dungeon Defenders (also new-ish, but great co-op) Team Fortress 2 Sid Meier's Civiziation V (vs. but it can be pretty fun and it handles it well, could probably do older ones as well) Left 4 Dead, Left 4 Dead 2 Guild Wars 2 (this game is actually great to play with anyone and will lower levels so that one player can be ahead then come back and play with the lower leveled players without blasting through everything)
Gamecube (probably the best system for co-ops): Final Fantasy: Crystal Chronicles (need GBAs for each player) Legend of Zelda: Four Swords Adventures (or anything with Four Swords in it, also needs a GBA for each player) Animal Crossing (while having to take turns, the game is great for having two or more people actively playing and sharing the same town) Super Smash Bros. Melee (already mentioned)
Wii (if you get an older one, it can play Gamecube games as well): Animal Crossing: City Folk (same as the Gamecube version, but better) Super Smash Bros. Brawl (a few more co-op specific game modes)
Nintendo in general: Any Kirby game
Xbox 360: Every Halo game (co-op on storyline and online allows two players under one name)
Yeah i've been thinking about Portal for a long time, i heard it is really good!.. left 4 dead we played,guild w is for online play right? ...yes i really liked Lineage II and wanted to play in coop too, but our internet conection here in dorm suxx..sooo
I don't know what systems you have. So I'm just gonna list all the co-op games I've played. I'm.. kind of in the exact same boat, except I'm the girlfriend.
Kirby:Epic Yarn Kirby: Return to Dreamland Rayman Legends Resident Evil (5&6) Resident Evil: Umbrella Chronicles Resident Evil: Darkside Chronicles Castle Crashers Bionic Commando: Rearmed (1&2) New Super Mario Brothers (only if you have a strong relationship) Lego Batman (1&2) Trauma Center (whatever they have on Wii now) House of the Dead (any, recommend Overkill) Otomedius (I couldn't actually play this, but it's got co-op) Metal Slug (any) Mario Kart (any) Super Smash Bros. Melee Mario Galaxy (1 only, and you have to take turns)
If you guys like fighting games, that's a whole other list Sorry this is so Wii-heavy, they just tend to release the most co-op games for that system.
we are limited to PC only and we actually bought wii sensor with all other stuf so we can play emulated games on PC, but they are running so slow so we are kinda limited on PC only! Also we played Metal Slug long time ago, great game..maybe we can play it again! I am actually hardcore gamer, so i basically know most of the games i spend my life actually working in Internet cafees and Pc gaming clubs, but now i am older so i kinda lost that string! I Love resident evil but i can't run it on my pc! also just to mention that we are currently at Thailand and we are using not very good internet connection so online coop is not an option!