HTML Editors change very fast so at any given time its worth looking at all the options and seeing which one is the best fit at the time.
TinyMCE is my current favourite HTML editor. It comes pre-packaged for ASP.NET but expect to have to re-build and debug it as its been a long time since it was updated. I am happy to email you my current build if it would help. Also as of the last build there is an error with the adding tables in Chrome. The fix for that is here
The YUI Editor is OK for many uses, and certainly worth considering if you already have YUI installed. Last time I checked it required more work to get some common functionality working such as view/edit raw HTML.
FreeTextBox used to be good. But lack of support puts this to the back of the queue. Looking at the website it could be that has changed as it looks like its undergone a revamp.
FTP version 3.1.6 (and earlier) has a bug in Firefox when used within
an IFRAME that the main page never completes loading - which is a
problem if you use the onload event. This appears to be more of a
problem with Firefox but the quick fix is to replace in
withthis.designEditor.document.open();this.designEditor.document.close(); // Hack to ensure iframe doc is closed correctly