What kind of forum software do you like?
I believe this forum uses bbPress. It's a mod of WordPress, a popular blog software. Anyway, I like to mess around with XenForo and MyBB. However, I am not especially prejudiced against any brand, though vBulletin has left a bad image in my mind (spam, bad graphics). However, the problem with VB could be the owners.
I think this forum works/looks well. I mean, I think there is a rep about bbPress being cheap, but really, any negative stuff about free forum software is either coming from developer snobs, people who aren't willing and/or smart enough to make adjustments, or just the ignorant.
OK, myself, I don't really want to fool with forum software so I'd rather have XenForo - despite the cost.
I found that my favorite tends to be Xenforo though IP.Board is probably a close second since that what I used when I was just getting into the forum game.
I do love how integrated this Wordpress forum software is though, makes it easy to maintain a website and a community. I can imagine it has its limitations though.