Textpattern
Textpattern is an elegant content management system that is free, open source software for Web designers, developers, publishers and bloggers. It is flexibility and extensibility. It has a powerful, sophisticated engine that can be infinitely tuned to suit whatever type of web site you can imagine. See more about Textpattern on their features page.
b2evolution
b2evolution includes all the features of traditional blog tools, and extends them with evolved features such as file & photo management, advanced skinning, multiple blogs support as well as detailed user permissions. See more about b2evolution on their feature summary page.
WordPress
WordPress is a state-of-the-art publishing platform with a focus on aesthetics, web standards, and usability. WordPress is free. For more about Wordpress see their About page.