Mephisto is a blogging platform, competitor to WordPress but on Ruby on Rails.
LifeType is a multi-user, multi-blog blogging platform. It started in 2003 and is up to date as at Mar 2008. Seems to support all the major features.
b2evolution is an open source PHP / MySQL blog platform. It seems to be more basic and less polished than the likes of WordPress, MovableType or Habari.
Serendipity is a PHP / MySQL blogging system. It uses the Smarty template engine.
Habari is an open source PHP / MySQL blogging platform (like WordPress).
A list of free / open source blogging platforms on Wikipedia. Handy. :)
Blojsom is a java blog platform. Multi-blog, multi-user blogging platform. It powers Apple's Mac OS X Tiger Server Weblog Server.