1. Wordpress.org
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.
http://wordpress.org

2. Wordpress.com
Express yourself. Start a blog.
http://wordpress.com

3. Typepad.com
TypePad is the premier blogging service for professionals and small businesses. TypePad hosts many popular blogs and small business websites. The service is easy to use and enables you to create a blog in minutes.
http://www.typepad.com

4. Movabletype.com
The perfect platform for building easy-to-maintain blog, web, and social media sites.
http://www.movabletype.com

5. Apache Roller - open source Java blog server - Apache Roller
Apache Roller is a full-featured, multi-user and group-blog server suitable for blog sites large and small.
http://roller.apache.org

6. Textpattern.com
A flexible, elegant and easy-to-use content management system. Textpattern is both free and open source.
http://www.textpattern.com

7. blosxom - the zen of blogging
Blosxom (pronounced "blossom") is a lightweight yet feature-packed weblog application designed from the ground up with simplicity, usability, and interoperability in mind.
http://www.blosxom.com

8. Serendipity Weblog System
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.
http://www.s9y.org

9. Slashcode: Slash Open Source Project
Slash is the source code and database that was originally used to create Slashdot, and has now been released under the GNU General Public License. It is a bona fide Open Source / Free Software project.
http://www.slashcode.com

10. PyBlosxom
PyBlosxom is a lightweight file-based weblog system. The project started as a Python clone of Blosxom but has since evolved into a beast of its own. PyBlosxom focuses on three things: simplicity, extensibility, and community.
http://pyblosxom.sourceforge.net

11. Lifetype.net
LifeType is an open-source blogging platform with support for multiple blogs and users in a single installation.
http://lifetype.net

12. b2evolution
b2evolution is a powerful blog tool you can install on your own website.It 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...Not to mention third party plug-ins!
http://b2evolution.net

13. Wonko.com Poseidon Page
Poseidon is a simple, minimalistic weblog system written in PHP. It is intended to be small, fast, and elegant. The core software provides the basic functionality for a weblog with content items and user comments, while a fully-featured plugin architecture allows for easy expansion and customization.
http://wonko.com/poseidon

14. Byteflow Blog Engine
Byteflow is a blog engine, written on Python, using Django. Why should you choose it over competitors? It has very clean codebase and developers, which are struggling to keep it so. It has a lot of cool features, which you can't get in other blog engines or will get with difficulty (consider feed by union of tags, eh?).
http://byteflow.su

15. Ideafry.com
IdeaFry is a totally free blog hosting service that allows you to create your own blog using WordPress Multi-User edition, or to import your Blogger, TypePad or LiveJournal blog.
http://ideafry.com