I personally prefer the PHP boards over ASP. Some good free ones are:
1) phpbb.com
2) invision forum (i believe that is what it is called)
I good paid one is Burning Board 2.x. It is almost as powerful as vBulletin but much cheaper. vBulletin costs $160 while Burning Board $50 (there is even a $25 license). We ran that till we decided to upgrade to vBulletin.