So the people over at WordPress have two new releases:
WordPress MU is an official branch of WordPress that is designed for managing and hosting thousands of blogs instead of just one.
bbPress is forum software with the WordPress touch, and developed by the same folks. It has some pretty cool features, such as tagging, RSS feeds, Akismet spam protection, AJAX interaction, but the team focused the most on creating something fast and light.
Here is the WordPress Development Blog with more information on the issue as well as the links for downloading the software for both releases.