The htmlarea seems to work with Mozilla and Gecko derived browsers as well as IE. It also seems pretty easy to add it to a site.
I can't really say why someone would use it for anything serious, textareas are still unfriendly even if WYSIWYG. It might be nice if you could select a file to upload, then put the contents into a text field in the database. Then you could use your favorite editor, save your work, spell check, etc.
Nicer still would be a simple upload tool, where you could map local files to content, maybe keep track of what you changed via an mtime or whatever. Hit one button and update.
The same format could be used to upload a zip file: read the mapping file and update the content.
