I really like Fireboard component for Joomla. It's quite nice. I also liked SMF back in the day (Simple Machines Forum). If you aren't using Joomla, it really isn't that much work to set it up so you could use it just for your forums (and maybe some other useful Joomla comps as well)
Fireboard:
Best of Joomla! The #1 joomla templates, resources and extentions
SMF:
Home of SMF: Free PHP and MySQL forum software
Both of these are FOSS.
I also found this:
ForumMatrix - Compare Them All
A comparison of forum software. Haven't tried it so YMMV.
Cheers,
-n8