WordPress is a popular open source blogging application written in PHP.

Multilingual, multiuser multi-blog engine.

AJAXPress is a web 2.0 blogging system written in Javascript with a PHP backend.

Classy news/weblog tool.

The original opensource weblogging and journal software.

bBlog is a powerful, elegant personal publishing system that is a flexible but simple way to blog that works for blogging beginners, and can grow into a more advanced user's needs.

Serendipity is an open source weblog/blog system that is standards compliant, feature rich and fully PHP5 compatible.

BLOG:CMS is the most complete, feature-packed, personal publishing system on the market, including a state-of-the-art weblog, forum (PunBB), wiki engine (DokuWiki), news aggregator, and photo gallery (singapore).

Feature ritch weblog, d-board, live webcam (option, and requires 3th party software), calendar, poll system, photogallery, smileys, search engine, 80% customizable and many more... most pages and modules can be switched on and off, custom text fields

Simple PHP Blog (SPHPBlog) is a dead-simple blog. It doesn't require a database, uses flat text files, and looks nice.

LifeType is an open-source blogging platform with support for multiple blogs and users in a single installation.

Pivot is a web-based tool to help you maintain dynamic sites, such as a blog.

Chyrp is a blogging engine designed to be very lightweight while retaining functionality. Driven by PHP, MySQL and AJAX, Chyrp has an excellent theme and module engine, enabling total customization.

LightPress is a frontend for the popular WordPress blogging platform. A frontend is the code sitting between the db and your blog's readers, which manages all user interaction and displays the relevant content appropriately formatted.

PHP/MySQL blog application featuring user comments, Linkdump with hitcount and custom dynamic pages.

Wheatblog is blogware: it's a web-based content management system for maintaining online blogs, journals, news pages, and about anything else you can think of writing down. It allows reader interaction with a comment system, and generates valid RSS 2.0 fe

Habari is a next-generation blogging application that uses PHP5, PDO, and your choice of SQL database. Habari is strongly object oriented, and implements the full suite of the Atom Publishing Protocol.

Simplog provides an easy way for users to add blogging capabilities to their existing websites. Simplog is written in PHP and compatible with multiple databases. Simplog also features an RSS/Atom aggregator/reader. Powerful, yet simple.

The open source microblogging tool that powers identi.ca.