Plunker (GitHub: ggoodman / stsh, License: MIT) by Geoff Goodman is a web application for editing web snippets. It's inspired by other popular services like jsFiddle, but has Node source that you can fork, and has an interface more suited to editing multiple files. It has all the obvious features like syntax highlighting, using the venerable Ace editor. The interface has been built with Twitter's Bootstrap, so it also serves as an example of a Bootstrap web application.
I often find myself needing to travel time, so rather than hacking Date I'd prefer to use mocks through Veselin Todorov's timekeeper (License: MIT, npm: timekeeper) library. Time can be frozen by using timekeeper.freeze(date). Once the normal passage of time is required, the fourth dimension can be corrected simply by calling timekeeper.reset().
The only caveat is setTimeout and setInteval won't work as expected until timekeeper.reset() is called.