XDForum
XDForum was the first forum plugin for WordPress, and for quite a while, the best. It's development was discontinued because I did not have the time in the past to keep up with it, and the other developers that worked on the system also were unable to commit enough time.
If you need a forum for your WordPress, I recommend you take a look at Simple:press > http://simplepressforum.com/. Simple:press also has an import script from XDForum if you are looking for something to migrate to from an old XDForum installation.