I have used this forum in my new site: http://www.EquiPortal.co.uk
The site is all about horses, so we have equine news, directories that only include horse sites, horse photo albums etc.
We have used the forum in a couple of interesting ways (I think):
1) The login for the forum is actually used for the whole site, so there are Login + Register options on the homepage.
2) When we show news items, there is a "discuss" link. Selecting this link automatically starts a new topic in the News forum, quotes the news item and allows you to make a posting against it. The idea is to easily allow people to chat about news items. I hope to add the "discuss" link to other items around the website, so the forum becomes the core component.
3) The homepage has a gadget which shows the latest activity on the forum. If you are logged in it also shows your posts, your subscribed topics etc.
Now the challenge is to attract equestrian people to the site. I don't think a forum encourages people to register until it has a good number of posts, which is a bit of a catch 22.
Anyway, thanks to PunBB for the software.