FCKeditor is the most feature-filled web-based rich text editor I have encountered so far. It requires no installation on the user’s computer, just JavaScript. Being an XHTML-compliant editor with compatibility for Internet Explorer 5.5+ and Gecko browsers (Mozilla/Firefox/Netscape), it beats the would-be competitors by a mile. It has the features of a fully-fledged word processor, so many that an entire CMS could be based around this editor.
The best part of all is that FCKeditor is open source, i.e. free! Needless to say, FCKeditor will definitely find its way into my future web applications.