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MOOdalBox: A modal box (inline popup), used to display remote content loaded using AJAX, web 2.0 style, written for the mootools framework.
It can be used to display some help document, an extra options page, a registration form, etc. It eliminates the need of classic popup windows, that may get blocked by popup blockers.
It's originally based on the excellent code behind Slimbox, so it inherits most of it's qualities / functionality. Compressed, it's only 4.2 Kb (about 25 Kb, including images, CSS and a light version of mootools 1.0).

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jQuery UI is a set of themable widgets and interactions, built on top of the jQuery JavaSctipt Library, that you can use to build highly interactive web applications.

The core of the library revolves around different mouse interactions, namely drag and dropping. sorting, selecting, and resizing.
On top of the core interactions are built a number of reusable widgets, including accordions, date pickers, dialogs, sliders, table sorters, and tabs.

Finally, the library is finished off with a few effects that you can use to liven up your site, specifically magnifier and shadow.

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MyGWT is an open source Java library for the Google Web Toolkit. The library helps developers easily create compelling desktop like AJAX web applications. No messing around with HTML, CSS, or images, all MyGWT widgets look great out the box. To see MyGWT in action take a look at the Explorer Demo.

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jQuery is a fast, concise, JavaScript Library that simplifies how you traverse HTML documents, handle events, perform animations, and add Ajax interactions to your web pages. jQuery is designed to change the way that you write JavaScript.