Centrify Express is the No. 1 choice of IT professionals for Active Directory-based authentication and single sign-on to cross-platform systems. Centrify Express is not only the quickest and easiest solution to use and deploy for integrating Linux and Mac systems with Windows, but delivers more functionality and more to upgrade to when compared to other free offerings.

And best of all — it is free!
http://www.centrify.com/express/free...-linux-mac.asp

I'm definitely gonna be testing this out very soon