I use to visit more forums than I do since I started this one.
I visit occasionally:
http://forums.servermatrix.com/ (My data center forum. I'm a moderator here.)
http://forums.olternit.com/ (My web host forum)
http://www.hosthideout.com/ (Web hosting forum)
http://www.sitepointforums.com/ (Web master forum)
But mainly... I just visit here.