I use Infopop’s eve. Works nicely. They have alot of features including a very complete user control panel and administration panels. They will even host it for you if you need that kind of option. It isn’t open source though which for some is a set back. My board has over 10,000 active members on it and it functions flawlessly. But they are not cheap.
here’s the link:
http://www.infopop.com/
There’s another forum software kit that is open source for around $300 that I hear is excellent, but I don’t remember what it is. I’ll look around. Also do a google search on forum software. You should come up with some options there.