I couldn't find any that I liked and which I felt were appropriate for my site (a church website, where most users have not used discussion boards before). I wanted something which would be simple for non-computer experts to use, rather than lots of features, so I ended up writing my own (which I intend to release soon). What sort of system are you looking for?

You can see the discussion board on our site. All output is via templates, so appearance is completely configurable if you know html.

Update: URL now fixed - thanks Corion