Over the years I have seen too many phpbb forums get hacked to recommend it. I fully understand that all software has security issues. But when you see phpbb forum after forum get hacked, there is something wrong. And during the mean time, smf, ipb and vbulletin are standing strong. There is something wrong when a "certain" forum software has more security issues then other forums. My vote goes to SMF. The support forums are ok, some of the mods are ok, and some mods crash your site. mybb - it has too many limitations and issues. I just came from the mybb forums, and there have been several threads posted in the last week about email problems. Plus, its limited on the number of modifications - as compared to smf. vbulletin is my preferred forum software, but the question is about free software. If you were to go back in time, say 5 years, phpbb ruled the free forum market, but not any more.
phpbb it is quite good developed and secure, there are many free scripts, themes etc for them, language packes and much more, as you see many people agree with phpbb.
That's mostly related to the new version released just one month ago. It's an incredibly complex and feature rich upgrade. Authors need time to create plugins and themes. I admit myself that it has issues but 1.4.1 was released about 2 weeks ago with 70 bugs fixed and 1.4.2 should be soon enough with about 80+ more bugs fixed that SHOULD mostly complete the release and it's major problems. The rate of new bugs has greatly slowed down the past week. Even PHPBB3 took a year or more to be in beta. Yet Mybb 1.4x has released just one month ago and it's still 10x better than phpbb3 now. Mybb 1.4x has an entire new admincp. One that looks great and has many amazing functions. The front end has also lots of goodies that make SMF and phpbb3 seem like 2002 scripts. It is without doubt the more feature rich free forum system. Just some kinks need to be ironed out. Also I understand complaining about the quantity of add-ons and plugins BUT...that's partially because so much is built in. Lots of ajax and controls. phpbb3...you need to immediately install at least the quick reply. What a freaking joke. SMF looks like crap and doing mods for their system isn't nearly as easy as mybb which has a full-featured plugin system along with hooks all over to get the job done WITHOUT any core code modifications.
Thumbs up for phpBB3 . Customizing it is a breeze. And the best part, the development team at phpbb are very active.
I've been a fan of phpbb, but after using vbulletin, I can't go back to phpbb. It's incredibly hard to add mods to phpbb3, and once you do, you can't really apply any security patches to it(without having to reinstall all mods). Save your money, and spend it on vbulletin.