Here is an interesting
competitor to vBulletin. I'm looking at it for some work based projects..
It's main feature is that it uses Ajax so that everything happens in real time. To use an example based on this site: If I posted this message while you were viewing the lounge, you would see the "new message" count update automatically. Also as new threads were being added to a hot discussion topic they would get added to your screen in real time. No need to keep hitting F5 (or refresh) all the time.
Oh and before someone asks,
no I am not suggesting LoT switch to this program. It's still in beta and has quite a way to go development wise, but it's still interesting.