Hey Toddskins: It was with msucianscollaboration.com - I just went back over there and signed in and found that my account is still alive! Surprised. Unfortunately, the forums look the same as they did there so many years ago.
Either way, it's basically a somewhat customized SimpleMachines forum, but you can go to the Collab Search on the left pane and easily see what parts people are looking for with a little icon for each part for each post - go to the post - chat, read what the songwriter is looking for, listen to other submissions, and submit your own "audition" for the piece. It is DAW agnostic. People just provide "drumless" (or whatever) tracks in wav or mp3 that you download and then send back a rough mix and/or the parts/stems or whatever they might request.
I used to have a good bit of fun, being a disconnected hobby musician. Not a lot of productivity really, but some fun and a few completed tunes in the end - some actually turned out pretty decent.
Hope that helps!