GitHub pages host only static HTML pages. No server side technology is supported, so Node.js applications won't run on GitHub pages. There are lots of hosting providers, as listed on the Node.js wiki.
App fog seems to be the most economical as it provides free hosting for projects with 2GB of RAM (which is pretty good if you ask me).
As stated here, AppFog removed their free plan for new users.
If you want to host static pages on GitHub, then read this guide. If you plan on using Jekyll, then this guide will be very helpful.
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