Okay, I just added Scheme to the list of Lisp dialects we'll be talking about. For Common Lisp, I added a series of forums for implementation-specific discussions. This could be anything from developer-related stuff to "how do I do sockets on implementation <foo>."
Unfortunately, I'm much more familiar with Common Lisp implementations than Scheme implementations. Schemers.org points to this list of implementations at schemewiki: http://community.schemewiki.org/?scheme ... mentations
Which of these would be the most used and most active? My guess would be PLT, MIT, Bigloo, Gambit, and Scheme48. But that's just my view of Scheme world from Common Lisp world and it's undoubtedly wrong.