Co-op game recommendations anyone?

Resident Evil 5 was the best co-op I ever played with my friend.

If you liked L4D 1 and 2 go for Payday: The Heist.

Sanctum is really great for multiplayer co-op.

Shoot many robots is a very fun game for co op in my opinion but I have only played the xbox trail version. I love the saints row games, you should play those in co op. Saints row 2 is $15 on steam but saints row the third will most likely cost around $40.

Nation Red isnt half bad

looks like you answered your own question ☺

Rage has limited co-op that looks promising but haven't tried it yet and it's currently $30 but I'm expecting a nice price with the Summer sales (fingers crossed)
It's liked about as much as it's hated, but Spiral Knights is at least worth a look for the price (free) with a group, it is quite fun - unless you hate it, then it probably blows
F1 2011 lets you co-op a full season as team-mates - sorry, probably turned off the path there

MMO:Maplestory
Strategy- League of Legends
Racing-Need for speed world
FPS- Combat Arms

Splinter Cell 3 and 4 have Exellent CO-op if you like stealth games
also first shank game have a complete co-op campaing

PD: BUY BORDERLANDS 2 NOW

PD 2: Your Avatar is pure Badassness

Revelations 2012

Saints row: The third

Some games others have not mentioned yet.

Dead Island (melee based FPS), Jamestown (Local only Top Down shooter), Lara Croft Guardian of Light (great co-op twin stick shooter), Dungeon Defenders (co-op action-rpg / Tower Defense), Titan Quest (Diablo-ish action/rpg). Also, you might as well preorder Torchlight 2, as its going to be pretty awesome.

Saints Row: The Third is great fun in co-op (if you like the humour, it's very "down-to-earth" to use polite words). Me and my girlfriend loved most of it, except for some stupid things with the AI. GamersGate have a -50% off promotion at the moment, though I don't know how long it will last.
I haven't played Saints Row 2, but I've heard equally good stuff about it.

Payday!

I hear Sven is an excellent coop, but haven't played it meself.

I think you should try Dungeon Defenders

• Serious Sam HD: The First Encounter and The Second Encounter are two of the best co-op FPSs I have ever played. If you like old-school titles of the genre, like Doom or Quake, this one will satisfy your needs.
• Since you said you liked Borderlands and Left 4 Dead, you should also enjoy Dead Island, although I believe Dead Rising 2 might be better if you're feeling like killing piles of zombies in bizarre manners. However, I have only tried that one in singleplayer, so I can't really confirm that.
• Resident Evil 5 totally rocks in co-op. It's really fun and especially great to play with a human partner instead of the dumb AI. Besides, it's quite challenging in the hardest difficulty setting and the numerous unlockables (including weapons, infinite ammo for each gun, outfits and collectibles) will give you excuses to play through the game several times.
• If you're into RPGs, there's also Torchlight 2, which is looking great and you can preorder for cheap, and World of Warcraft, the best MMO I have played so far. In WoW you can form a party and quest together, it's a blast.

I guess that covers pretty much everything I can think of. I hope this helps!

PS: Saints Row: The Third may be very fun indeed, but it gets old kind of fast and has a couple of lag issues while playing on LAN (at least I had them with my brother), so I'm not sure I would recommend it. By the way, Saints Row 2 is rather good, but it's a lousy port and some things in the game feel broken.

Try Nation Red, its fast-paced zombie topdown shooter. Try demo first cause its very specifical gameplay

Going on a date, Thats co-op.. Or even multiplayer.

Saints row 3 aint bad. (sales are soon)

Saint's Row 3,Portal 2,Splinter Cell Conviction.

sex and after let dude play alone

If you enjoy isometric style RPGs, then I recommend getting Sacred. It's pretty awesome, and it's only $10. BUT, I believe you'll only be able to play through LAN, since the servers for the game don't exist anymore. I mean, I guess you could try to use Hamachi if you weren't going to be playing in the same place, but it will probably lag like crazy.

Other games I recommend:

Quake II
Rainbow Six Vegas
Neverwinter Nights Complete
Dungeon Siege Complete I-III

And if you're into MMOs;
Guild Wars Trilogy
Dungeons & Dragons Online (F2P)

Play Magicka.
DO NOT play Bunch of Heroes, it's fucking awful.

APB:Reloaded

Magicka, Torchlight (and upcoming Torchlight 2)