Cross-referencing commits in github
The answer, so us lazy people can find it without following the link to the Autolinked References and URLs section of the GitHub help:
User/Project@SHA
For example:
mojombo/god@be6a8cc1c1ecfe9489fb51e4869af15a13fc2cd2
Short SHAs work as well (as long as they are unique):
mojombo/god@be6a8cc