I can second YABB (http://yabbforum.com) in terms of ease of installation, configuration, and features. We recently installed this for some collaborative work in my organization and it seems very useful.

Of course, it's Perl. I have yet to glance at any of the code though, so I can't vouch for quality/elegance/etc. The person who set it up used the basic flat-file mode, we need to switch to a database backend soon (which should be very doable).