GitHub Downloader
I have created a new gem, GitHub Downloader, which does basically that: it downloads the source code from GitHub repos, packed in zip files.
It's open sourced, of course. You have in there the usage instructions too.
I needed an script for doing that, and well, maybe it works for other people so there you have. It's on RubyGems already, give it a try if you find an use to it!