TinyMCE es lejos,mejor editor WYSIWYG que he visto, opensource, gratis.
Carácteristicas:

• 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.

Lo recomiendo.