Beehive is a Free and Open Source forum system. The key feature which sets Beehive apart is its user-friendly split-pane view, showing the thread list on the left and messages on the right.
Beehive also supports many other unique user-centric features: targetted posts to users and/or other posts; HTML posting; a user relationship system; powerful forum wide and per-user word filter (supports regex); flexible polling allowing public & private polls, grouped answers, and different result formats; plus much more.