Why would you want to write your own WYSIWYG editor?

Rich Editor Controls that you can use with ASP.NET

HTML Editors
As near as I can tell, all of these work in-browser and produce HTML or XHTML.

• RadEditor (Telerik). AJAX-enabled.
• WebHtmlEditor (Infragistics). Part of the NetAdvantage for ASP.NET suite (I think).
• Cute Editor (CuteSoft)
• FreeTextBox (FreeTextBox.com). Free for basic edition. Source code available (for a price).
• Peter’s Textboxes (Peter Blum). Suite of enhanced text box controls.
• UltimateEditor (Karamasoft)
• Asbru Web Content Editor (Asbru)
• ASP.Net XHTML WYSIWYG Editor (MoreNet)
• AspLib Component library (Astron Digital). Includes a WYSIWYG editor, among other things.
• ASPxHtmlEditor Suite (DevExpress)
• ComponentArt Editor for ASP.NET AJAX. (ComponentArt)
• EditLive! (Ephox)
• eWebEditor (eWebEditor)
• eWebEditPro (Ektron)
• InnovaStudio WYSIWYG Editor (InnovaStudio)
• FCKeditor (FredCK.com)
• Html TextBox (ActiveUp). Part of the ActiveUp.WebControls suite.
• Obout Editor for ASP.NET (Obout)
• pinEdit (Pintexx)
• PowerWEB TextBox for ASP.NET (Dart).
• RichTextBox (aspControls)
• RichTextEditor (Syncfusion)
• RTE ASP.NET Control. Released under an MS-PL license on CodePlex. Includes source.
• SmartText (Z-SoftNet)
• soEditor (SiteObjects).
• SPAW Editor v.2 (Solmetra)
• TinyMCE (Moxiecode). JavaScript based; open source under LGPL.
• Ueditor (Yardi). Free; source and restribution for a price.
• WebHtmlEdit Control (JitBit Software)

Word Processing, RTF, PDF, and more
These variously support other formats, notably non-HTML (e.g. RTF) and sometimes PDF.

• ASP.NET Rich Text Box (TX Text Control). Supports DOC, DOCX, RTF, TXT, HTML.
• Aspose.Editor (Aspose). Many formats, including HTML.
• TE Edit Control (Sub Systems). RTF editing.
• TextDynamic Server / wRTF2PDF (WPCubed).

Other/Not Sure
I’m not sure how exactly these fit into the picture; they’re listed at least in one location as being ASP.NET editors.

• Community Editor (BigByte). Desktop editing, it says; possibly not in-page HTML editing? Appears to be free.
• DevEdit NX (Interspire). Not 100% clear that it supports ASP.NET.

More Information

• A similar list is available at 123aspx.com.
• Daniel Walzenbach published a list as well in December 2007. With pictures!
• Scott Mitchell has an article on using FreeTextBox.
• “Building a WYSIWYG HTML Editor” A two-part article by Mitchell Harper. I’m pretty certain that this is for Internet Explorer only