Arguably the most popular free software forum solution available, phpBB, has just been bumped to version 3.0. It is a major new release of the forum software and includes many new features whilst also modernising the core architecture.
From the press release:
phpBB, the leading open source forum and online collaboration system announced today the availability of phpBB Version 3.0. This release includes enhanced collaboration features, better security and delegated administration features, extended support for open source and commercial database management systems, and optimisation for mobile devices and search engines. phpBB is available at no cost, released under the GNU General Public License.
Some of the new features in 3.0 versus the 2.x series include support for subforums, a feature which many have been crying out to get implemented for some time, attachments, birthdays and other collaboration features. phpBB also ships with a fresh new default theme.
phpBB is licensed under the GPL, runs anywhere where PHP and a compatible database system runs, and can be downloaded from the phpBB Downloads page.
You can find out more information about phpBB 3 from the release announcement and press release on the site.