Jquery Zoomer
Zoom up your iFrames
Mincss
Tool for finding out which CSS selectors you're NOT using.
Stats.js
JavaScript Performance Monitor
Showdown
JavaScript port of Markdown
Retina.js
JavaScript and LESS helpers for rendering high-resolution image variants
Leaflet
JavaScript library for mobile-friendly interactive maps
Less.js
Leaner CSS
Normalize.css
A modern, HTML5-ready alternative to CSS resets (as used by Twitter Bootstrap, HTML5 Boilerplate, TweetDeck, Soundcloud, and many others).
URI.js
Javascript URL mutation library
Dropbox JS
JavaScript client library for the Dropbox API