For the googly-eyed Git goon and Ruby-smitten gem jabber, GitHub has been the place where the cool kids are storing their code. It's essentially a Web interface to control your Git-controlled source code, but all the social and Webbish add-ons make it a Radical Ninja way to handle your code.
You could, for example, fork Ruby on Rails right now. If you wanted to screw with it in ways too terrible to be seen, you can pay them to make your fork private. It's a good business model: Everyone's gotta play out in the open if they want it for free.
Today, GitHub released its code in an installer (built by also radical company, BitRock). That installer can be run behind your firewall. You can finally run GitHub in your server room. It's quite a killer app for the Git community.
If you're new to Git, you should read up on how merging works, as it's a bit different from Subversion.