Rte_Multi
   Open source WYSIWYG component that can replace all textarea fields in every module for PostNuke, eNvolution and similar CMSes (core and 3rd-party). Only works for MSIE 5.5 SP1+ under Windows [Open source, GPL].
   http://sourceforge.net/projects/rtemulti/

Spaw
   Full-featured, fully customizable, multilanguage, skinable web based WYSIWYG HTML editor control. PHP and ASP .NET versions available. IE 5.5+ required, other browsers see just a plain textarea [Open Source, GPL].
   http://sourceforge.net/projects/spaw

Toko Content Editor
   Toko Content Editor is a free multi language and multi-user web editor. Commercial version also available.
   http://toko-contenteditor.pageil.net