Matt Mullenweg has released Wordpress 2.0.5, an open source blog engine based on PHP and MySQL. 2.0.5 fixes a dozen or so assorted bugs. WordPress is published under the GPL.
I use WordPress to power The Cafes and Mokka mit Schlag. It's got a lot to recommend it including the user interface and themability. However it has some serious problems with HTTP, XML, and security that the developers are in denial about. It may (or may not) be the best open source blog engine available today, but it's certainly not even close to the best one that's possible.
