I've tried quite a few, as I've been in forums since the mid 90s.
MyBB I haven't tried in a few years, but I hear it has come a long way. When I tried it (probably in '07 or '08?), the features were very limited.
I've had phpBB2 for ages on a few sites, and have also used phpBB3. Seeing how slow their development cycle is, I would not recommend phpBB even to a competitor's forum. Wouldn't wish that headache on anyone. phpBB3.1 is still versions behind where vB was a few years ago. Their support forum is horrible.
SMF had a nice following, but development is essentially dead. They've been in version 2.0RC status for years now. Behind the scenes, the development team had split up and were warring with whoever operated the website IIRC. It was promising, but having been burned by being stuck on a very heavily modified phpBB2, which took a good five or six years to make it to the next major version, I won't get myself stuck again. I have one or two sites still using SMF that need to get migrated, and soon.
I can't recommend bbPress at all: I've tried a few versions, and the backend has way too many shortcomings for a busy forum operation. I liked that it integrated somewhat with Wordpress (in fact, it is a WP plugin now). It just lacks way too much and IMHO, they are reinventing a wheel that has already been over-invented dozens of times.
MyBB appears to have an active and enthusiastic development team, and a healthy following. I have a feeling we'll be seeing more of them in the future. Having been with two dead-end free forums (phpBB and SMF), that would be my own top "freebie" pick.
