hunt : Library to detect when DOM elements become visible and disappear on scroll.
ClippyJS : Add Clippy or his friends to any website for instant nostalgia.
nlp_compromise : nlp_compromise does NLP in the browser. It's a handy, and not overly-fancy tool for understanding, changing, and playing with english. We've also got ...
ProseMirror : In-browser structured text editing component. A toolkit for building rich-text editors on the web.
TraceKit : Not all browsers support stack traces on error objects, but TraceKit squeezes out as much useful information as possible and normalizes it.
Rainbow is a code syntax highlighting library written in Javascript.
It was designed to be lightweight (1.4kb), easy to use, and extendable.
It is completely themable via CSS.