You could try Citadel...

citadel.org

Webcit is the web front end.  You can try out a live site
by visiting uncensored.citadel.org and getting an account.

A bit more than a message board and is a sign up process, but
the sign up process is user driven.  It has some nice features
like being able to replicate between citadel setups and other
items like groupdav and imap access.

Kelly