http://federal-repositories.herokuapp.com/ (you'll have to do a one-time authentication against your github account)
Note that the original version at http://gsa.github.com/federal-open-source-repos/ is still live but now hits the API rate limits of GitHub for unathenticated users. This is a good problem! It's because there's such growth in government GitHub usage.
Also note sister projects at www.govcode.org and http://ada.mbecker.cc/USAxGITHUB.
Note: By contributing to this project, you agree to license your work under the same terms as those that govern this project's distribution. See license.txt for more information.