SmartMarkUP is a flexible & lightweight JavaScript library that turns any textarea into a markup editor.
It can be configured to edit HTML, CSS, XML, Wiki syntax & BBCode.
The library is unobtrusive & degrades gracefully if JavaScript is disabled.
SmartMarkUP has a simple and powerful API. Styles can be changed, new markups can be implemented, extra buttons and submenus can be created & more.
It also supports keyboard shortcuts for easier usage & has add-on support like color picker, search-replace, special character picker (and new ones can be created).
Compatibility: All Major Browsers Except IE6
Website: http://www.phpcow.com/smartmarkup/
Demo: http://www.phpcow.com/smartmarkup/examples
Download: http://www.phpcow.com/smartmarkup/downloads/