phpbb2, vanilla, e107 - depends what you need it for. www.opensourcecms.com has loads you can preview.
Well the big question is do you want a commercial or free one? If it's commercial I really like vbulletin it has constant development along with probably having the best feature set. For free I'm partial to phpbb although it seems to be showing it's age so hopefully phpbb3 helps with that.
SMF is definatly a great script. Am using it on my 2 forums (after using phpBB) and I definatly recommend it.