Blackbird is a very cool JavaScript debugging tool with a nice interface to display and filter debugging information.
http://www.gscottolson.com/blackbirdjs/
Treesaver is a JavaScript framework for creating magazine layouts.
http://treesaverjs.com/
Bibliotype is a simple framework based on HTML, CSS, and JS for rapid prototyping.
http://craigmod.com/bibliotype/
Highcharts is a JavaScript library that is written in pure JavaScript and is used to produce interactive graphs.
http://www.highcharts.com/* * *
Jstat is a statistical toolkit developed using JavaScript that allows you to perform advanced statistical operations without the need for a specialized statistical language.
http://www.jstat.org/
Highlight.js is a JavaScript library for highlighting program code and is easy to use.
http://softwaremaniacs.org/soft/highlight/en/
This fast, lightweight library allows ie6-8 to support CSS3 Media Queries, a feature that allows Web pages to support the display of small-screen devices.
Http://filamentgroup.com/lab/respondjs_fast_css3_media_queries_for_internet_explorer_6_8_and_more/
Underscore is a practical JavaScript ToolPak that provides programming support similar to the prototype functionality, but does not extend to JavaScript-built objects.
Http://documentcloud.github.com/underscore/
Popcorn.js is a HTML5 video framework that provides Easy-to-use APIs to synchronize interactive content.
http://popcornjs.org/
Slickgrid is a library for building simple, fast, flexible tables.
Https://github.com/mleibman/SlickGrid/wiki
The JavaScript Infovis Toolkit is used to create visual data for interaction.
http://thejit.org/
Handlebars is a JavaScript page template library that helps you easily build semantic templates. Refer to Thinkvitamin's article getting started with Handlebars.js.
http://handlebars.strobeapp.com/
Backbone.js is a JavaScript MVC application framework for Key-value binding and custom event handling through models.
http://documentcloud.github.com/backbone/
Openfaces is an open source framework that contains the ajax-powered JSF component library, an AJAX framework, and a client-side validation framework that derives from Quipukit and adds new components and features.
http://www.openfaces.org/
D3 allows you to bind any data to the Document Object Model (DOM), and then apply data-driven delivery to the document.
http://mbostock.github.com/d3/
JavaScript is an event-driven language, and this visual event library is used to help you track events.
Http://www.sprymedia.co.uk/article/Visual+Event
The tempo rhythm is a small JSON rendering engine that allows you to build data templates using pure HTML.
http://twigkit.github.com/tempo/
The library that implements the letter separation effect preserves the attributes that the text can be selected.
http://manufacturaindependente.com/colorfont/
Less is a library that expands the CSS to have dynamic language features.
Http://designshack.co.uk/articles/css/using-less-js-to-simplify-your-css3