For the uninitiated, "GitHub.io" refers to GitHub Pages—a hosting service that takes code from a repository and turns it into a live website. When you combine this with the word "game," you unlock a universe of thousands of browser-based, open-source, and completely free games.
Go to GitHub.com and use the following search string: language:javascript game html5 stars:>50 game github io
It represents a return to the roots of the internet: creative, shareable, and free. Whether you want to merge numbers to 2048, hunt zombies with your keyboard, or build the pyramids in a civ clone, the GitHub.io universe has you covered. For the uninitiated, "GitHub