It's like jsfiddle but your code is stored in github gists. It's entirely open source and except for the oauth handshake to log in it's entirely a static website.

I don't really care if anyone else uses it but I used it a ton. Of course I worry that github will take it down or I'll hit some other limit but it was fun to build and your gists won't be lost.

Also https://jsbenchit.org which similarly, uses gists and is open source, and, again I don't really care if anyone but me uses it but I do use it.