BuddyPress.org

It works for the main blog too, thought the search function isn’t all that useful given there would only be a single blog but all the other functions work with the main blog.

All you’d need to do to get this setup is to change the default role for the main blog from subscriber to contributor and disable blog registration. Also, you may want to hack at the code to remove things such as the blog directory, the list of user blogs, etc. but this is optional.