https://sourceforge.net/projects/wordpress-mu. I had worked with hedir to make the multi user wordpress work.
There are many more things left like,
A proper user management. Themes edit (which is a big challenge)
I am sure this is much better than http://mu.wordpress.org. If you are interested in working with me on this project please contact ajinimc(at)users.sourceforge.net. I will get back to blogging next month.
I need to make it work again, I work over 10 hrs for office + 4 hrs of traveling + additional work. I need to get some more motivation to finish it. It will take me 10 hrs of coding. For last few months I have started doing external integration for a easy updates. Internal integration is a pain when security patches are released.