The VOIDRUNNER'S CODEX is LIVE! Explore new worlds, fight oppressive empires, fend off fearsome aliens, and wield deadly psionics with this comprehensive boxed set expansion for 5E and A5E!
I used to use ikonboard as others have recommended before me. It's decent, free, good looking, and doesn't require mySQL. However, you will need some webspace somewhere that supports PHP.
If you are running a windows server, check out the asp.net forums forums. They are free and really fast. I don't think they are highly customizable though.