Rich Text Editor TinyMCE

TinyMCE is a platform independent web based JavaScript HTML WYSIWYG editor control released as Open Source under LGPL.

Try it http://www.tinymce.com/

For CRM 2011 RU 12,13,14 try below link

http://chamarairesh.blogspot.com/2013/07/rich-text-box-with-crm-2011-rollup-12.html