WordPress MU is multi-user version of the famous WordPress blogging application. It is ideal for people wanting to offer a hosted version of WordPress, but due to its complexity installation and maintainance is not supported in the same manner WordPress is.
Using WordPress Multi-user edition you will be able to people be able to sign up for a new blog and have them securely manage their templates and settings without affecting any other users. Only one blog per user is allowed, but you can have unlimited users, and you can have multiple users on a single blog.
With the excellent work of Donncha this project is just getting off the ground, so check back often for updates.