[jaz]

It's not exactly a single place, but this page [0] lists the Github organizations for many federal agencies. It's not comprehensive, notably it's missing the NSA organization [1].

[0] https://code.gov/agencies

[1] https://github.com/nationalsecurityagency

[xenophonf]

It's missing https://github.com/niaid/ and related NIAID repositories, too.