This is the code and data behind http://develop.github.com.
All content can be found in the _posts/ directory.
Install Jekyll an rake (both require Ruby):
gem install jekyll rake
Once you've done that, run rake to compile the site:
rake
This should start a web server at http://localhost:3000 which is now serving your site, updating whenever you make changes.
To contribute to the develop.github site, you can fork the repository, push your changes into it and create an Issue: http://github.com/develop/develop.github.com/issues
If you want to run the site on GitHub pages for testing you can push your changes into the 'gh-pages' branch, rather than the 'master' branch of the remote repository.
$ git push origin master:gh-pages
That command will push your master branch to the 'gh-pages' branch of your fork. Then Pages will serve the site for you under:
http://schacon.github.com/develop.github.com
(Replace 'schacon' with your username)