opensource rich text editor for asp.net

"CKEditor" is the best WYSYWIG editor that can be used for asp.net. For more details check here.

In addition AjaxToolkit also provides a HtmlEditorExtender which takes advantage of HTML5 and supports IE 6 and later and other leading browsers. Check more details here.

Edit
. Refer this article for an excellent example of using HtmlEditorExtender. You might face some configuration errors (i.e. Unrecognized element 'sanitizer'.    ) when setting up this control.

Change web.config as follows to overcome this error.
  <configSections>
 <sectionGroup name="system.web">
      <section name="sanitizer" requirePermission="false"
               type="AjaxControlToolkit.Sanitizer.ProviderSanitizerSection, AjaxControlToolkit" />
    </sectionGroup>
  </configSections>

The first entry in <system.web> section shoud be chaged as follows
<trust level="Full" />
  <sanitizer defaultProvider="HtmlAgilityPackSanitizerProvider">
      <providers>
        <add name="HtmlAgilityPackSanitizerProvider" type="AjaxControlToolkit.Sanitizer.HtmlAgilityPackSanitizerProvider"></add>
      </providers>
    </sanitizer>
You'll be need ed to add a reference to all three assemblies contained in the folder: SanitizerProviders.dll,and HtmlSanitizationLibrary.dll which can be found in the AjaxControlToolkit source.

Cheers,
Samitha