Humanized Messages is a javascript-based system for non-model notifications, from an idea by Jef Raskin, as relayed by Aza Raskin and adapted for jQuery

stream.js is a tiny stand-alone Javascript library that unlocks a new data structure for you: streams.

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

A compiler for the Mustache templating language

Backbone.js gives structure to web applications by providing models with key-value binding and custom events, collections with a rich API of enumerable functions, views with declarative event handling, and connects it all to your existing API over a RESTful JSON interface.

Easily create apps using the web technologies you know and love: HTML, CSS, and JavaScript.
PhoneGap is a free and open source framework that allows you to create mobile apps using standardized web APIs for the platforms you care about.

Teach your forms some manners!

HTML5Forms.js is a JavaScript polyfill that implements a subset of the HTML5
Forms module in all browsers. The script will only add support for the
different parts of the module when there doesn't exist a native
implementation.

Web Forms 2.0 Cross-Browser Implementation

Online code editor and playground.

A passive reconnaissance tool inside the DOM (experimental)

A super micro tiny DOM library for authoring HTML5 mobile web applications.

Javascript MPEG-1 Audio Layer III (mp3) and ID3v2 decoder

JavaScript is a class-free, object-oriented language, and as such, it uses prototypal inheritance instead of classical inheritance. This can be puzzling to programmers trained in conventional object-oriented languages like C++ and Java. JavaScript's prototypal inheritance has more expressive power than classical inheritance, as we will see presently.

localStorage wrapper for all browsers without using cookies or flash. Uses localStorage, globalStorage, and userData behavior under the hood.

Activates media queries in IE6-8

This is a javascript library (with frontend) for sound effect generation.

History.js gracefully supports the HTML5 History/State APIs (pushState, replaceState, onPopState) in all browsers. Including continued support for data, titles, replaceState. Supports jQuery, MooTools and Prototype. For HTML5 browsers this means that you can modify the URL directly, without needing to use hashes anymore.

CSS3 pseudo-class and attribute selectors for IE 6-8