TinyMCE

TinyMCE is a platform independent web based Javascript HTML WYSIWYG editor control released as Open Source under LGPL by Moxiecode Systems AB. It has the ability to convert HTML TEXTAREA fields or other HTML elements to editor instances. TinyMCE is very easy to integrate into other Content Management Systems.

TinyMCE Features
* Easy to integrate, takes only two lines of code.
* Customizable through themes and plugins.
* Customizable XHTML 1.0 output. Block invalid elements and force attributes.
* International language support (Language packs)
* Multiple browser support, Mozilla, MSIE, FireFox, Opera and Safari (experimental).
* PHP/.NET/JSP/Coldfusion GZip compressor, Makes TinyMCE 75% smaller and a lot faster to load.
* You can easily use AJAX to save and load content!

http://tinymce.moxiecode.com