I've been happily using Gitosis for quite a while now. It's like Github without the web interface (so it manages central repositories and user authentication).

I recall it took a bit of finagling to set it up, but I haven't had a single issue with it since. It's also cool that the configuration files are stored in a Git repository (once you try it, it will be clear how that works).

http://scie.nti.st/2007/11/14/hosting-git-repositories-the-e...