css3-mediaqueries-js : make CSS3 Media Queries work in all browsers (JavaScript library)
impress.js : It's a presentation framework based on the power of CSS3 transforms and transitions in modern browsers and inspired by the idea behind prezi.com
Crafty : JavaScript Game Engine, HTML5 Game Engine
SoundManager 2 : Using HTML5 and Flash, SoundManager 2 provides reliable cross-platform audio under a single JavaScript API.
dancer.js : dancer.js is a high-level audio API, usable with both Mozilla's Audio Data API and Webkit's Web Audio API with flash fallback, designed to make sweet ...
Add Clippy or his friends to any website for instant nostalgia.
