I'm personally an Invision Power Board fan, although I also like phpBB.
Regarding the licensing issue: It is my understanding (based on the documentation to the IPB sitting on my computer) that you could run an IPB for free, but would not be entitled to any support or upgrades.
Just a note about ProBoards: On the online "NationStates" game I play, many
people have had their passwords stolen by unscrupulous forum administrators, as it's not difficult for a ProBoard admin to get one's forum password. So unless you
really trust the administrator(s), I recommend against registering on ProBoard forums.