GrimForum is an Open-source forum project by HSMX web design. It was written in the winter of 2002 by Matt Squirrell, while he was waiting for his girlfriend to come back from university. It is driven by PHP 4.0 on a MySQL server, and it's fast, highly customisable and very, very pretty.
B4.1 is the first public release, so there a doubtless a few bugs left to iron out. You can take a look at HSMX's working GrimForum here, or look at another use of the GrimForum here.
If you want to use GrimForum B4.1 on you site, then good news! It's free, as everything on the web should be. You'll need your own web space, and a .php MySQL database to store the information in. To download the setup kit, visit the download page.
If you don't have .php space or you need someone to host your database, then HSMX web design is the obvious choice! Visit the contact page for more details.