What is your favorite GitHub repository?

The list of lists. Actually I have several bookmarks like that. It's a treasure chest of good links. https://github.com/jnv/lists

If you look for something more specific the list pointed me to https://github.com/bolshchikov/js-must-watch

When I want to see what good Python looks like, I browse the Django source code. I like the style, the structure, and the approach to documenation.

https://github.com/django/django

I like the activity on rails. And the comments on rails :)