TraceKit : Not all browsers support stack traces on error objects, but TraceKit squeezes out as much useful information as possible and normalizes it.
Parsley.js : Javascript forms validation.
Powerful, UX aware & Dead simple.
meSing.js : meSing.js is a JavaScript singing synthesis library that uses the Web Audio API's DSP capabilities in conjunction with the meSpeak.js speech synthesis...
hunt : Library to detect when DOM elements become visible and disappear on scroll.
tarteaucitron.js : The european cookie law regulates the management of cookies and you should ask your visitors their consent before exposing them to third party service...
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 a modest, though ambitious plugin ecosystem.