Three main ones and at least a few others. Different forums have different focuses and not all users are on each forum.
SSF/Slingshot Forum - http://slingshotforum.com/
Rebel Slingshot Forum - http://rebelslingshotforum.freeforums.org/index.php
Trumark has a forum, but I don't think there's been any new posts on it since last spring. Melchior Menzel has a forum for his website "Slingshots of the World" but I didn't see much traffic there last spring when I was joining forums. I hear tell of a Jack's Shed forum in the UK, but haven't registered there at all.
Melchior's site has some good information - http://slingshot-shooting.de/
Nico has good information on hunting projectiles on his blog - http://nicos-resorterablog.blogspot.com/
YouTube has a lot of good videos about how to slingshot. Joerg, Simpleshot/Flippinout, Bill Hays, pfshooter, Charles Morgan, Gamekeeper John, and many others.
It's like a buffet. Try a whole bunch of stuff and take extra helpings of the stuff you end up liking. Just be safe and responsible.