There are a few other brands on there, including Walmart and Amazon:

https://github.com/walmartlabs

http://www.readwriteweb.com/archives/walmartlabs_kosmix.php

https://github.com/amazonwebservices

I can imagine Goldmans are using Github to acquire talent and new ideas. I am sure they have some complex problems and technology that needs fixing.

Stack Exchange had a discussion on this last year:

http://programmers.stackexchange.com/questions/61062/why-sho...