*Free updates coming
Download WPMU DEV Multi Domains. Multi-Domains from WPMUDEV is the only plugin which works flawlessly with the lastest version of WordPress. It is BuddyPress compatible, has a single signon and even a multi-subdomain feature (blog1.subdomain.site.com, blog2.subdomain.site.com). There is a free alternative, but it has not been updated for the latest version of WordPress (the author is not currently available to do it) and it is not as full featured as the wpmudev plugin. This plugin is developed by WPMU.
WPMU DEV Multi Domains Features: