Well, there's Arcanum, but it's buggy as all heck and I've had no luck getting the online aspect to work. Can't play Oblivion or any other Elder Scrolls games multiplayer, they're strictly one-player games.
Vampire: The Masquerade Redemption can be played in a cooperative mode of sorts. The idea there is that there are seperate stories made specifically for multiplayer mode, and adding new stories to it isn't that difficult from what I've heard (only played it, never tried making my own stuff). There is a dungeon master-esque mode that lets you more or less control every tiny aspect of the game too, but like I said, I never did much with it.
Vampire: The Masquerade Redemption can be played in a cooperative mode of sorts. The idea there is that there are seperate stories made specifically for multiplayer mode, and adding new stories to it isn't that difficult from what I've heard (only played it, never tried making my own stuff). There is a dungeon master-esque mode that lets you more or less control every tiny aspect of the game too, but like I said, I never did much with it.