Of course, i should talk about RTS. Real Time Strategy games are usually pretty functional for providing some good cooperative gaming. The reason is simple: they almost always have AI ennemies available, and the most recent games offer even to set some characteristics of the AI (such as rusher behaviour etc.) and they allow for players to work together in a number of ways.

In Starcraft for instance, there was a mode where allied plyaers actually begun at the exact same spot and had to work together to make their cities grow, including the possibility of controlling each others' armies. This opened the way for some uneasy situations, but most of the time, on a LAN cooperative players can agree on what the plan is and what to do. That was very effective, and demanded a good deal of cooperation, even if, at the beginning of the game some players had to patiently wait for the other(s) to be set so that he in turn could get his race going.

Age of empires 2: Age of Kings was similarly fun, with the difference that the difficulty could be set (starcraft was set on "mean" and you couldn't do anything about it; Warcraft 3 suffers from the same problem that can really be nasty on new players that don't want to go through the single player campaigns).

Currantly, i am playing Rise of Nations, and here appears a new philosophy: the rules to win can be totally customized: for instance, you can decide that the game should be won totally based on economics results. Since my wife is on the peaceful side of games, she can now enjoy the game without having to wage war.

RTS, generallly speaking, are therefore pretty good for cooperation. Specially for the part where you can exchange ressources and send your army to protect your allies. I like also the idea of armies attacking from different angles a single ennemy... death by crushing is most likely.