CORTEX COMMAND! Of all the local co-op games I've played, this one takes the cake.

I use a service called Tunngle (Google it). It lets you play LAN games over the internet. Very handy, especially since it opens up a ton of games to play with friends you otherwise would have to play alone (or with someone on the same network). It's free, but you can pay for a Premium account and get extra features.

pokemon!

If you're talking about LAN, and not split screen, then Resident Evil 5 and Borderlands 1 & 2 proved to be quite fun.

Gears Of War 1 :D Pretty awesomely fun!

Steam:

• Rayman Origins

• Shank

• Trine

• Street Fighter x Tekken (if you can call fighting along with your friend a co-op)

Non-Steam:

• Jazz Jackrabbit(I played only the second one, it had a coop for sure)

• Dynasty Warriors 4 or 6 (hard to get), Samurai Warriors 2 is pretty much the same, but less popular. There's also Warriors Orochi, it combines both, I think. Good co-op games, I had lot of fun playing DW5 on PS2 with my brother.

• I've heard there was Metal Slug collection for PC, but better grab some MAME emulator to play them, it's hard to get. Greatest co-op games I know.

• X-Men Legends II: Rise of Apocalypse - 4 player co-op, if I remmeber corretly. Great game.

Orcs must die 2

Warhammer 40k Dawn of War II

trine1 is great ! havent played trine2 yet but that one is probably great also :D

I like Rayman Origins for local coop

• Dirt3
• Dirt3 Showdown
• Resident Evil 5
• Family Guy : Back to the Multiverse
• The First Templar
• Kane & Lynch: Dead Men
• Kung Fu Strike: The Warrior's Rise
• Rocksmith (lol yes)
• Dead Pixels
• Dungeon Siege 3

some coop games that havent been listed so far (i hope so at least)

Nice little troll link to log out with. Reported for not activating your Humble Bundle #3 win, as well as Tryst, and Oozi: Earth Adventure.

You should try Shank 1 and 2, F1 Race Stars ( so much fun in those games ), Trine 1 and 2... Mad Riders also, Worms, Dogfight 1942, and ofcourse some sport games :)

Starcraft, Diablo II, Titan Quest, Halo (not sure), Rainbow 6 Vegas I & II (not sure)...

EDIT: I should read the OP more often

heros might and magic

The Cave supports co-op on one PC.

Beat Hazard

Command & Conquer: Red Alert is pretty fun to play with a friend.

BloodBowl, it has both Local Co-op so you can be 2 on the same computer and it has LAN and Internet play, it's damn awesome and me and my brother play it a lot and have a good laugh and hate from it :)

Age of Empires 2 - Best LAN game imo

Super Mario X
It's for free, but you must find it on google. Official site is deleted, because Nintendo threatened to court. This Mario is EPIC, trust me, and u can play with 2 players in one time.

payday - the heist

RAYMAN ORIGINS!