Take a look at http://sourceforge.net/projects/freetextbox/. Though this is
not a component for richtext. If you require the user to edit text, change
fonts, insert hyperlinks etc. this will do the job.
If .NET is installed on the client then you can build some C# control
that is (similar to an applet) running on the client. This is not
feasible on the Web in general where you can't expect the client to be
IE with .NET installed.