Beehive is simply the best forum software I have seen.
One of my favourite features is the way it remembers what you've read; not got time to read everything now? not a problem, read some now, come back later to read some more, it dosn't mark everything as read just because you've visited the forum.
Reply to specific posts within a thread, email notifications, PMs, HTML posting with extensive checking to stop the script kiddies messing with your forum.
you should check it out yourself, you won't be disappointed, this is way better than a lot of commercial stuff available and best of all - it's free!