I'm only aware of two collabs - a fantasy game, not set in the Forgotten Realms; and a modern game, set in a Miami Vice like place.
I've got everything for both of them. (I know, you're really surprised.) There was quite a bit done for the first, but basically the persons who had volunteered to do the writing never stepped up, so slowly but surely, I stepped in with original ideas, that I meant as a seed to get people thinking, but aside from one other person doing levels, it was just me with a group of people watching what I was doing.
I probably should release it as an unfinished project, or maybe, finish it, or, and this may be crazy, open it back up as a collab.
The other collab went the same route, but didn't go on for as long, so didn't gather as much art and such. Sigrid helped out quite a bit and Rotsel had a lot of good ideas, but we never decided on anything.
Noone ever responded to my questioning about interest in doing a super hero collab, so I tried to work on a world conversion, calling it 'Hero Craft'. But, I lost steam and haven't touched it in a while.