RadEditor for ASP.NET AJAX

RadEditor is not simply an HTML¹ Editor. It is what Microsoft chose to use in MSDN, CodePlex, TechNet, MCMS and even as an alternative to the default editor in SharePoint. Whether you need a mere Textbox with Google-like spellchecker, or a Word-like content authoring environment, the result is the same: clean XHTML output, fast rendering, widest cross-browser support, and tons of features:

Out-of-the-box XHTML-enabled Output...
Unmatched Loading Speed and Performance
Microsoft Word-like Spell-checking
Seven Ways for Pasting from Word
Multilevel Undo/Redo with Action Trails
Extended Functionality Through Integrated Controls

Browser/OS	Windows	Mac OS	Linux
Internet Explorer	6.0+	-	-
Firefox	1.5+	1.5+	1.5+
Google Chrome	0.2+	5.0+	5.0+
Opera	9.0+	9.0+	-
Safari	3.0+	3.0+	-
Netscape	9.0+	9.0+	9.0+

^1.The computer language used to create world-wide-web pages which are read by browsers.