PS2 Coop games

Baldur's Gate: Dark alliance
Baldur's Gate: Dark alliance 2
Tales of the Abyss (coop at battle only)
Champions of Norrath: Realms of EverQuest
Champions: Return to Arms
Mystic Heroes  
X-Men Legends 2: Rise of Apocalypse
Marvel : Ultimate Alliance gameplay co-op
Dynasty Warriors 4
Star Wars Battlefront
Star Wars Battlefront II
Neo Contra
The Legend of Spyro: Dawn of the Dragon
Ratchet: Deadlocked
ONIMUSHA: DAWN OF DREAMS
Gauntlet Seven Sorrows
Warriors Orochi 2
Justice League Heroes
tales of symphonia (coop at battle only)
ff9 (coop at battle only)
Tenchu: Wrath of Heaven
Marvel Ultimate Alliance
Guitar Hero
Gauntlet Seven Sorrows

http://ca.ign.com/articles/2010/05/07/the-best-ps2-co-op-games


http://www.co-optimus.com/system/6/playstation-2.html


http://www.cheapassgamer.com/forums/showthread.php?t=183288


The best coop games for the PS2 are Hack n’ Slash (so if you don't like hack n' slash you are probably out of luck):
Baldur’s Gate Dark Alliance 1 and 2 (the most fun)
Champions of Norrath/Return to Arms (fun)
The Dynasty Warriors/Samurai Warriors games (fun)
Star Wars Legos I&II (fun)
Mortal Kombat: Shaolin Monks (fun)
Xmen Legends I & II (unopened on shelf)
Hunter Wayward (not fun)

FPS coops:
Timesplitters Future Perfect (the best) and Timesplitters 2 (fun)
Darkwatch (fun)
Half Life (includes a few missions called Decay which are coop) (OK)
Medal of Honor: Rising Sun (OK)
Rainbow Six 3 (not my favorite)
Delta Force Black Hawke Down (haven’t played it enough to form an opinion)

Stealth coops:
Splinter Cell Chaos theory and Double Agent (fun).

Horror coops:
Obscure and Obscure 2 (fun when playing coop)

Platformer coops
Tak the Great JuJu Challenge (fun)
Cookies and Cream (fun)
Wallace & Grommit Curse of the Wererabbit (not as bad as it sounds)
Avatar the Burning Earth (really bad game).