b2evolution
b2evolution is probably the most comprehensive blog engine you can find! It includes almost any feature you could expect from a blog tool, and more !
Homepage: http://b2evolution.net/
Documentation: http://b2evolution.net/man/index.html
Support Forum: http://forums.b2evolution.net/
feedonfeeds
FEED ON FEEDS, your server side personal RSS (and Atom!) aggregator.
Homepage: http://minutillo.com/steve/feedonfeeds/
Support Forum: http://sourceforge.net/forum/forum.php?forum_id=282735
Serendipity
Serendipity is a PHP-powered weblog application which gives the user an easy way to maintain an online diary, weblog or even a complete homepage. While the default package is designed for the casual blogger, Serendipity offers a flexible, expandable and easy-to-use framework with the power for professional applications.
Homepage: http://www.s9y.org
Support Forum: http://www.s9y.org/forums/
WordPress
WordPress is a state-of-the-art semantic personal publishing platform with a focus on aesthetics, web standards, and usability. What a mouthful. WordPress is both free and priceless at the same time.
Homepage: http://wordpress.org/
Support Forum: http://wordpress.org/support/