Chosen is a javsacript plug-in that makes long, unwieldy select boxes much more user-friendly. It is currently available in both jQuery and Prototype flavors.
Enquire.js is a JavaScript library I created for dealing with media queries in JavaScript.
Helium - javascript tool to scan your site and show unused CSS
Repository of utility libraries that can be used with the Google Maps JavaScript API v2.
A Modern, Lightweight Open-Source JavaScript Library for Interactive Maps