Xinha - Open Source Online HTML Editor
Xinha (pronounced like Xena, the Warrior Princess) is a powerful WYSIWYG HTML editor component that works in all current browsers. Its configurabilty and extensibility make it easy to build just the right editor for multiple purposes, from a restricted mini-editor for one database field to a full-fledged website editor. Its liberal, BSD licence makes it an ideal candidate for integration into any kind of project.
The one line explanation is that Xinha turns <textarea> form fields into HTML editors.
We can do better than a one line explanation though, because you can add a basic Xinha editor to your site with one line!
For more information, downloads and documentation please see the Xinha website