BuddyPress.org

I’ve cobbled together my own one WP-install, multi-domain solution using the resources below as a starting point. But, this is only for single-version WP, not WPMU. It is also for development purposes only using vhosts on my local machine.

Here are the articles to get you started:

http://me.mywebsight.ws/2006/08/11/host-multiple-wp-sites-on-one-installation/

http://www.ryanmcdonnell.com/multiple-blogs-one-wordpress-install-zero-code-changes/

http://www.bos89.nl/how-to-multiple-blogs-one-wordpress-installation/