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...
script.aculo.us : script.aculo.us provides you with
easy-to-use, cross-browser user
interface JavaScript libraries to make
your web sites and web applications fly.
howler.js : howler.js is an audio library for the modern web. It defaults to Web Audio API and falls back to HTML5 Audio.
DOMAssistant : The modular lightweight JavaScript library, with CSS selectors, event handling, AJAX and complete Unicode support
Anime.js : Anime.js is a fast, multipurpose and lightweight JavaScript animation library with a simple, yet powerful API.
It works with CSS properties, SVG, DOM ...