Open Source Blog Software

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...

LifeType

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

Movable Type

Movable Type is an enterprise grade blogging platform.

NucleusCMS

A PHP/SQL blogging tool which resides on your webserver. Includes multiple weblog capabilities, and RSS syndication.

Wordpress MU

WordPress MU, or multi-user, is designed to do exactly that. It is most famously used for WordPress.com where it serves tens of millions of hits on hundreds of thousands of blogs each day.