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.
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.
Download
The latest version of WPMU can be found on the download page.
Support and Help
The support forum is the place to ask questions, but use the search function first as your question may already have been answered!