Well, I haven't played it, but Sacred is pretty much a straight Diablo clone, and is supposed to be pretty good. There was another Diablo-type game with an Asian setting as well, but I can't remember the name. Also Icewind Dale is pretty cool in multiplayer, but you gotta have a lot of patience. And even though I don't like it, you might want to give Neverwinter Nights a shot.... what else.. oh yeah, Ragnarok Online is supposed to be pretty fun.
Other than that, the upcoming Hellgate: London (developed by the original creators of Diablo) looks like it will kick all kinds of ass, but that's not due out for a while.
BTW, they're not multiplayer, but I would suggest trying out the games that inspired Diablo: Nethack, Rogue, Omega, etc. Those games are way rad.