The best cooperative multiplayer game I have found so far that I really enjoy is Monster Hunter Freedom. It is a really hard game to play by yourself but with a friend you can create a mule character that will help a lot. Only basic supplies can be given to friends so the challenge is not lost. There can be 4 players on a mission at once.

Some of the missions must be completed solo but it is not necessary as I have taken a second character all the way to hunter rank seven and still need to beat the lvl 3 for the solo section. There are many online walkthroughs that tell a person what missions are needed to go up in rank. The game is an indepth. Each mission is from 30 to 50 minutes long. Then there is also the garden and shops to make things interesting.

I believe MHF3 comes out in March. It should be better for those playing solo as they will have companions (like dungeon siege does... or at least that is what I understand)