VIA : .Net Adventures
FCKeditor this HTML text editor brings to the web much of the power of desktop editors like MS Word.
Damn Small Rich Text Editor - very small text editor.Built on top of well known JQuery library.
TinyMCE -Cross platform HTML WYSIWYG text editor .Supports templates, easy localizable and can be extended using plugins
HotEditor - browser-based rich text editor that provides an advantage in simplifying the workload and providing options for integration to forum software
WYMeditor - simple cross platform XHTML web editor
SPAW Editor is a web based in-browser WYSIWYG editor control enabling web site developers to replace a standard textarea html control with full-featured, fully customizable, multilingual, skinable web based WYSIWYG editor.