Bloggidity.com ask the question. I know well FreeTextBox you can so I checked HTMLArea and yes it has some good point over the first one.
First it's open to different languages, so not only .Net but PHP for example. It's also full compliant with XHTML.
I discover also something cool, the fact that you can use plugins to improve the tool. One of them (see the demo here) is really something I missed in FreeTextBox (and others). It's call Full Body and let you edit your HTML code with the Body tag, not without like others.
Amazing and you can also link to external stylesheets so then you really see your code as it's going to be publish.