WordPress  

WordPress is a state-of-the-art semantic personal publishing platform with a focus on aesthetics, web standards, and usability. Features: Instant publishing with no rebuilding - Full standards compliance - Integrated blogrolling features - Password-protected posts - Dead simple upgrades and installation - Dozens of styles and templates Multiple authors - Multiple categories - Sub-categories.

Hosting Requirements: PHP
Database: MySQL
Price: Free (GPL)

   pLog  

Blogging platform built with PHP, designed with maximum customizability, speed and ease of use in mind.

Hosting Requirements: Apache; PHP 4.1+
Database: MySQL 3.1.x+
Price: Free (GPL)

   Epanelapse  

Epanelapse is a PHP application designed to build and publish in weblogs, news sites, etc. It doesn't need MySQL. With Epanelapse you can build and update websites in instantly without any knowledge of PHP, HTML, etc. It allows comments, archive the news in different files, etc.

Hosting Requirements: PHP 4.2+
Price: Free