WordPress Multi User (WPMU) is beginning to establish itself as the standard in multi user blogging. While many projects might prefer a more stable or community based solution for pure blogging with strong open source credentials and an exciting future ahead this may be the answer you’re looking for.
pLog is a clear, simple and effective open source multi user blogging tool. With an established community and stable build this should be in consideration whatever application of blogs you are considering.
http://blogsavvy.net/multi-user-blog...gs-and-reviews