So far I can't find one. But you can google the list of forum in the internet with many visitors in it.
Every forum software has it's own way of managing URLs. Some has dynamic links and some has static links. Mostly these URLs are accessible for bots (Google, Yahoo, etc...) unless you surf each link using a web browser. One good way to find these is to find a RSS database where millions of forums are submitting their feeds everyday. You could find lots of RSS databases if you search.