Always lots to look at at
hotscripts.com
XMB,Phorum, as mentioned phpBB,Invision (nice, but I don't like their license)..
There was another really small forum package that I liked, but can't remember the name.. It certainly didn't have all the features of the big dogs, but was pretty slick and seriously fast. ack.. (maybe "Freeb")
Although k4 bb looks interesting with AJAX.
Best bet is to install some and see how you like them.