DOMAssistant : The modular lightweight JavaScript library, with CSS selectors, event handling, AJAX and complete Unicode support
polyfill : polyfill makes web development less frustrating by selectively polyfilling just what the browser needs. Use it on your own site, or as a service.
SoundManager 2 : Using HTML5 and Flash, SoundManager 2 provides reliable cross-platform audio under a single JavaScript API.
Tracery : A story-grammar generation library for javascript. Write grammar objects, get generative stories.
Workbox : JavaScript Libraries for adding offline support to web apps.
Add Clippy or his friends to any website for instant nostalgia.