Dropzone.js : DropzoneJS is an open source library that provides drag'n'drop file uploads with image previews.
Band.js : An interface for the Web Audio API that supports rhythms, multiple instruments, repeating sections, and complex time signatures.
Vex : Vex is a modern dialog library which is highly configurable, easily stylable, and gets out of the way. You'll love vex because it's tiny (6kb minified...
enquire.js : A lightweight, pure JavaScript library for responding to CSS media queries.
Autotrack : Automatic and enhanced Google Analytics tracking for common user interactions on the web.
Modest Maps is a small, extensible, and free library for designers and developers who want to use interactive maps in their own projects.