I use two forums, firstly i use Invision Power Board, i prefer the skins and the easy way of installing/upgrading, and just the overall look, i'm a member of many online forums and i generally stick around the IPB forums more.
Secondly i use Phorum, a nice small, easily skinable phorum for those sites that dont get a lot of traffic and have no need for the bloatedness that is IPB/PHPBB, i got it on my site at
http://www.echobeatmusic.com/phorum