I've just set up InvisionBoard on a new site that I'm developming and I must say that I'm impressed. Even though this is only the first release, it is a quality product. The lead developer was the original developer of IkonBoard. It is written in PHP and uses a mySQL database so it is not cpu-intensive. Almost every design element (e.g., graphics, colors, etc.) are under the control of the administrator through the Admin Control Panel. I've managed to tweak the script some to set up a mini-CMS for my new site (which I hope to have open the first part of July). If you're looking for a good free forum script, I'd suggest you check this out.

FYI, I have no affiliation at all with InvisionBoard.