? will show the help (a list of keyboard shortcuts)
t will let you search for a file in the repository, and jump to it
/ will focus the GitHub search box
. will open the current file or folder in GitHub's editor
In a file:
y will switch to the canonical URL (this is useful to get the current commit hash in the URL, so you can share the link, without worrying that the content may change in future)
b will open git blame for the current file
If you are viewing a file which uses tabs for indentation, and you want to reduce their size, add ?ts=2 or &ts=2 to the page URL