phpBB has been subject to numerous hacks and worm cracks in the last year, but it is free.
The development team has released fixes, but only after hundreds and hundreds of boards have been compromised. Those security issues have led RedsZone's host to ban phpBB from their servers.
vBulletin is very popular, resource intensive, on the expensive side and you're soaking in it.
I prefer Invision Power Board, myself.
If you want to "try before you install" http://www.opensourcecms.com/
has demos of several OS packages installed with open passwords for the back end. Over there, you can flip switches and kick the tires of these forums.