The whole "Wiki" feature comes to mind. Can I leave and take my project's wiki with me?
> git clone git@github.com:wicketstuff/core.wiki.git
More of a problem would be your issues.
With a few plugins and some hacking, you can get vim to just browse the wiki too. (I'm currently working on getting my scripts for this into a state that is useful).
Unrelated - there are some plugins to use the github api to browse tickets in vim too.
(note not all of these workflows make sense as a primary way of using them, but they are still useful at times).