If you want to boost your website's functionality, a rich-text editor is one of the most requested features — it's present in almost every CMS. Here are the 5 best JavaScript text editors, in my opinion:
1. TinyMCE — http://tinymce.moxiecode.com/
The most powerful and widely-used editor. Used by WordPress, Microsoft, Oracle, Apple, and Joomla. Supports multiple languages, AJAX integration, theming, and extensive plugin ecosystem. Somewhat heavier than alternatives, but incredibly feature-rich.
2. HTMLBox — http://htmlbox.thematicmedia.com/
Built on jQuery, easy to integrate with clear documentation. Small footprint and AJAX support. Great choice when you need something lightweight but functional.
3. markitup — http://markitup.jaysalvat.com/home/
A very flexible, developer-friendly plugin. Not a full WYSIWYG editor but an excellent toolbar plugin if you don't need all the bells and whistles. Supports keyboard shortcuts and Markdown/BBCode/HTML markup sets.
4. CKEditor — http://ckeditor.com/
An extremely powerful editor used on many major websites. Excellent documentation, keyboard shortcuts, AJAX support, extensive configuration options, and a large plugin library. One of the most professionally maintained editors available.
5. jWYSIWYG — http://github.com/akzhan/jwysiwyg
A simpler, lighter editor with AJAX preview and solid cross-browser compatibility. Good for projects that need basic editing without the overhead of TinyMCE or CKEditor.