In-browser code editor

Tinycon allows the addition of alert bubbles and changing the favicon image. Tinycon gracefully falls back to a number in title approach for browers that don't support canvas or dynamic favicons.
Alerts in the favicon allow users to pin a tab and easily see if their attention is needed.

Presentation framework based on CSS3 transforms and transitions.

D3.js is a small, free JavaScript library for manipulating documents based on data.

Library for hover/popup windows

JavaScript library

Code to check whether cookies are enabled in the user's browser

Javascript optimizer, library, templating system and style checker (linter)

Underscore is a utility-belt library for JavaScript that provides a lot of the functional programming support that you would expect in Prototype.js (or Ruby), but without extending any of the built-in JavaScript objects. It's the tie to go along with jQuery's tux.

Underscore provides 60-odd functions that support both the usual functional suspects: map, select, invoke — as well as more specialized helpers: function binding, javascript templating, deep equality testing, and so on. It delegates to built-in functions, if present, so modern browsers will use the native implementations of forEach, map, reduce, filter, every, some and indexOf.

Growl-like notifications in a webpage; jQuery-compatible

The JavaScript InfoVis Toolkit provides tools for creating Interactive Data Visualizations for the Web.

Processing.js is an open programming language for people who want to program images, animation, and interactions for the web without using Flash or Java applets. Processing.js uses Javascript to draw shapes and manipulate images on the HTML5 Canvas element. The code is light-weight, simple to learn and makes an ideal tool for visualizing data, creating user-interfaces and developing web-based games.

Processing.js is explicitly developed for browsers that support the HTML5

"Raphaël is a small JavaScript library that should simplify your work with vector graphics on the web. If you want to create your own specific chart or image crop and rotate widget, for example, you can achieve it simply and easily with this library."
Also: gRaphaël for graphs and plots

Various open source widgets for data visualization, including a very neat timeline widget

Tool that compresses JavaScript (and CSS, too)

JavaScript syntax highlighter