Apparently, none of the popular discussion forum software have this functionality. bbPress has this and i love it. in fact, i use a third party mod to show the latest ongoing discussions on the homepage of my forum powered by vBulletin. the basic idea is... new people landing on your forum can see the ongoing discussions without venturing into individual forums. and a returning user can quickly see the latest active threads on the forum's homepage itself without checking out the latest posts feature.