Apache Roller is a full-featured, multi-user and group-blog server suitable for blog sites large and small. Apache Roller is a Java web application that should be able to run on any Java EE server and relational database.
Roller's key features:
Multi-user blogging: can support tens of thousands of users and blogs
Group blogging with three permisson levels (editor, author and limited)
Support for comment moderation and comment spam prevention measures
Bloggers have complete control over blog layout/style via Apache Velocity-driven templates