While waiting for Diablo 3 to be released, maybe even later this year, you might want to crack out your old Diablo or Diablo 2 discs for some good ol' fashioned co-op gaming. Let me guess, you can't find them. Yeah, I had the same problem.
Good thing Amazon is running a killer deal right now on the Diablo Battle Chest. You get both Diablo 1 and Diablo 2 as well as the Lord of Destruction Expansion for Diablo 2. Top it all off with a Diablo 2 strategy guide and you'll be ready to take on the hordes of hell with your friends.