By "mainstream" I mean the versions put out by the original JForum team (Rafael Steil et al.) hosted at jforum.net. Their last version was JForum 2.1.9, and they are not working on that any more. They
are working on JForum 3, but that is far from being finished, and -according to Rafael- will not be finished any time soon.
The JForum 2 fork version hosted at Google Code is active. At the moment Andowson and myself are the principal committers to that. While I'm active here at JavaRanch, Andowson is not, so anything you want to bring to the attention of the JForum2-fork-at-Google-Code team would best be posted in the forums at andowson.com. While that fork hasn't seen a release in a while, and the forums aren't very active, the SVN repository has a number of
improvements over the released 2.3.4 version, and those will be part of the next formal release in the not too distant future.
Here in this forum at JavaRanch, most people seem to discuss the now dead 2.1.9 branch. That's why I direct people to Google Code and the forums at andowson.com if I notice they they're really talking about that version, or are looking for an actively maintained version.
(To complicate matters, these forums here run on JForum, and we have a forum about JForum, so some people think JavaRanch is in some way connected to the JForum effort - it's not, and JavaRanch takes no position on these matters. The JForum version that runs here is a heavily-customized version of 2.1.9, and is not available to the general public.)