I'm playing some co-op games with my friends now, but i don't know if you already tried them. Anyway the games that I'm playing now are Doom 3 (opencoop 1.1) and half life 2 (co-op MOD). those are the main FPS games but you could also try SWAT 4 or Rainbow Six Las Vegas. If you are looking for strategy games I would recommend Warcraft 3 (TFT) or C&C 3. Me and some guys from school are playing Age of Empires 2, also pretty cool. If you like the RPG elements in games then Dungeon Siege 2 might be a good one. Or if you want to go oldschool you could download a SNES emulater and play secret of mana with friends or something like that. It's a shame that many great single player games for the PC don't have co-op modes.