WordPress MU is an official branch of WordPress that is designed for managing and hosting thousands of blogs instead of just one. It’s the software that powers WordPress.com, for example. MU has been in heavy development for about a year now, and we’ve finally polished it up to a place where we feel like it’s ready for public consumption. Since setup is a bit more complex than the 5-minute install of regular WordPress, MU is best suited for a more server-savvy audience. You can download it on the WordPress MU site.