The HTML5 Shiv : The HTML5 Shiv enables use of HTML5 sectioning elements in legacy Internet Explorer and provides basic HTML5 styling for Internet Explorer 6-9, Safari...
Bliss.js : Want to use Vanilla JS but find native APIs a bit unwieldy? Bliss is for you
TraceKit : Not all browsers support stack traces on error objects, but TraceKit squeezes out as much useful information as possible and normalizes it.
Band.js : An interface for the Web Audio API that supports rhythms, multiple instruments, repeating sections, and complex time signatures.
PeerJS : Peer-to-peer data in the web browser.
PeerJS completes WebRTC as an API abstraction, connection broker, and binary serialization format
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.