GitHub works well with Tor except for the signup step (https://github.com/join).
GitHub employs a broken CAPTCHA that prevents signups over Tor. The CAPTCHA simply does not render properly.
The error message (after submit) is:
Unable to verify your captcha response. Please visit https://help.github.com/articles/troubleshooting-connectivity-problems/#troubleshooting-the-captcha for troubleshooting information.
Very frustrating for anonymous souls. It is unclear whether this is intentional or not.