SMF is a very popular - and therefore well supported - one, and it is free.
AskMeHelpDesk is run by
vBulletin, fyi.
Also, check with your web-host. If your host uses Fantastico, check there for what they offer for easy installs and setup.
(I don't know anything about IPB - except that I've heard a few complain that can be very resource intensive on servers)