PHPBB is widely used forum software and likely as good as free gets, if you don't mind the recurring hacker exploits and security issues. ;-/

PunBB is more secure and much faster than PhpBB. For a more radical option, have a look at Vanilla.

I will recommend phpBB as it has good mods. Many are developing mods for phpBB.

When I have to make such decisions I check their forums and the participation level. More members, more participation helps me decide for it.