meSpeak.js : meSpeak.js (modulary enhanced speak.js) is a 100% client-side JavaScript text-to-speech library
enquire.js : A lightweight, pure JavaScript library for responding to CSS media queries.
React : A JavaScript library for building user interfaces
Photon : UI toolkit for building desktop apps with Electron.
Band.js : An interface for the Web Audio API that supports rhythms, multiple instruments, repeating sections, and complex time signatures.
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.