Aurion: I played a lot of RPGs with some friends.
I suggest we make a list or something, not only for rpgs, but for every good old game out there you can play (in coop, that is). I know there are some lists out there, but often when i look for coop i come across games like Command & Conquer and so, which is multiplayer, but not coop.
So i'm posting every rpg i can imagine now, i know you posted a lot of them already.
Action-RPG/Hack&Slash
Diablo II [+ LOD]
Dungeon Siege (not that good imho)
Dungeon Siege 2 (see Dungen Siege)
Space Siege (uh..even worse, the gameplay itself. MP is ok if you look at what they did to that game :/)
Titan Quest [add on existing](if Diablo II is 100%, this is 80% or so. Fun, but not overwhelming)
Silverfall (Bugs, but still fun)
Silverfall - Earth Awakening (stand alone expansion, less bugs)
Sacred (it was fun a while, but didn't complete it.. its just to long if you want to do everything, and gets boring. Play it fast, then its more fun)
Sacred 2 (i only played the demo, and no multiplayer, but seems like Sacred)
"Real" RPG:
Neverwinter Nights (had LOTS of fun, with original campaign and several fan made modules, in multiplayer and singleplayer)
Neverwinter Nights 2 (i must admit, i didn't play it that much in mp yet, but i started singleplayer, seems solid enough)
Baldurs Gate (it's slower then NWN i would say, and i didn't like it that much, but it is ok)
Baldurs Gate 2 (like Baldurs Gate)
Icewind Dale (never played it though)
System Shock 2 (with coop mod/fan patch. Try it, its fun)
Tactical RPGs:
Fallout: Tactics (singleplayer was great, even if it was less rpg then Fallout 1&2, didn't play it in mp yet, but i would say it would be nice)
Jagged Alliance (i have to say, i didn't play JA (and JA2) really long, but a lot of people love it. At least for JA2 there is a mod out there afaik, who gives you some multiplayer/coop)
Jagged Alliance 2 (see Jagged Alliance)
This last categorie isn't that much role playing often, and multiplayer/coop isn't always that easy to achieve, but some elements are there/it's possible.
As i did not play all the games on my list, i can not assure you it has real coop, but they are supposed to have.
So far
Aurion
I am playing Sacred 2 on and off right now, but I could sure use some help with building a specific type of character. I'm having trouble figuring out what to take to maximize what I want. The games good, it's just really got a lot of depth, which is actually a great thing. I wish they had a character planner app like CoH/V does so I could play around with different skills and stats to see what the outcomes could be. It's tough because investing all that time in the game and messing stuff up is a possiblity, unless you can respec your character? Anyway, I really enjoy that game.