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.
Dotclear is an open-source web publishing software created in 2002 by Olivier Meunier. A one man's project at first, Dotclear soon gathered a team comprising different personalities with various backgrounds. The project's purpose is to provide a user-friendly tool allowing anyone to publish on the web, regardless of their technical skills.
b2evolution is an advanced weblog tool, i-e software allowing you to run your own blogs, newsfeeds or even photo stream. b2evolution is a medium scale CMS centered around the Blog concept, fully featured with multiple blogs, content/presentation separation, full user management, full internationalization (i18n), workflow management... and extending towards features normally provided by Wikis, Trackers or hierarchy driven CMSes.
Chyrp is a blogging engine designed to be very lightweight while retaining functionality. It is powered by PHP and has very powerful theme and extension engines, so you can personalize it however you want. The code is well-documented, and it has a very strong structure that's loosely based on the MVC design pattern.
PivotX is free software to help you maintain dynamic sites such as weblogs, online journals and other frequently updated websites in general. It's written in PHP and uses MySQL or flat files as a database. PivotX is also a great tool to maintain more complex websites as well. Its powerful core and flexible template system make it easy for developers to adjust and extend. Whether you want an easy-to-use, robust blogging tool or are looking to push things to the max - PivotX offers the best of both worlds.