[Izzmo]

I use to use Invision, but they turned greedy on us! Was awesome until they did so.

SMF (Simple Machines Forum) is fantastic. It originates from YaBB (yes, I'm an ol' timer) Perl forum. SMF is great, and like ryukenden said, "[it] is more secure."

Don't let phpBB fool you though. phpBB 3, the newest addition, is very reliable and secure, and includes so major updates to themes and the back-end which make it a good candidate once again.

I've used both phpBB and SMF in the past month, both of which I love. So I'm split.