The AJAX Control Toolkit is a joint project between the community and Microsoft. Built upon the ASP.NET 2.0 AJAX Extensions, the Toolkit aims to be the biggest and best collection of web-client components available.

Evaluant. State of the art mathematical expression evaluator.
DynamicControlsPlaceholder To create a control that behaves like a placeholder but additionally handles recreating dynamic controls on subsequent requests..
FCKeditor. This HTML text editor brings to the web much of the power of desktop editors like MS Word. It's lightweight and doesn't require any kind of installation on the client computer.
Prototype Provides class-style Object Oriented and AJAX, freely distributable under the terms of an MIT-style license.
script.aculo.us A collection of Web 2.0 style JavaScript libraries that help web developers to easily add visual and ajax effects to projects.
TinyMCE tinymce is a platform independent web based Javascript HTML WYSIWYG editor control released as Open Source under LGPL by Moxiecode Systems AB.