gelato cms is described as a tumblelog cms. It's based around the tumbr concept, you can easily post snippets. Various themes and plugins are available.
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.
Pebble is a lightweight, open source, Java EE blogging tool. It's small, fast and feature-rich with unrivalled ease of installation and use. Blog content is stored as XML files on disk and served up dynamically, so there's no need to install a database.
Apache Roller is a full-featured, multi-user and group-blog server suitable for blog sites large and small. It powers internal and external employee blogs and Sun, IBM and other companies as well as the JRoller.com Java blogging community.