WordPress MU is a multi-user, multi-blog version of WordPress and it is quickly gaining popularity. It allows you to run many blogs from a single installation of the WordPress MU software.
The WordPress MU homepage is located at http://mu.wordpress.org, and from here, you can access the latest version.
Although, WordPress MU re-uses 90% of the WordPress code, there are some differences between the two, mainly due to the fact that MU is meant to host multiple blogs simultaneously.
For example, if you were creating an XML sitemap plugin for normal WordPress, and created the file by default in the root of the blog, it would not work on MU, as each blog would be overwriting the sitemap every time it was generated.
Other differences come from the fact that WordPress MU is not always updated at the same time with WordPress. So if you use new features and API functions available in the latest version of WordPress, they may not work in the latest version of WordPress MU; that...