Just when I thought that forum software had dropped off the perch, I find Vanilla. Rocketing up the charts to #3 on Google (although I see that Yahoo! and Microsoft Live search aren't with the program) it's a refreshing change to the same-ness of phpBB and its derivatives, spiritually or otherwise.

It's built to be extensible, and from a quick look at the code, seems pretty well thought out. There are over 300 extensions registered to date!

As some have noticed, it can be thought of more as a live web/blogging tools rather than a forum, since it's permissions are quite granular and customizable.

Overall, it's still immature (most themes aren't cross-browser, for a start), but it has a lot going for it.