GitHub AE is currently under limited release.
You can navigate and track changes in the code in your files.
You can view raw file content or trace changes to lines in a file and discover how parts of the file evolved over time.
When viewing a file on your enterprise, you can press the "y" key to update the URL to a permalink to the exact version of the file you see.
You can download a snapshot of the code in your repository.
GitHub AE supports rendering and diffing in a number of non-code file formats.