markItUp! is a content editor plugin based on jQuery. It's not a WYSIWYG editor. Instead, it provides a lightweight, easy-to-use set of buttons, keyboard shortcuts, and other tools to add markup to your content.
More info on the markItUp jQuery plugin, and demos at http://markitup.jaysalvat.com/home/.
(Please also note that the excellent BUEditor module provides much of the same functionality, however, I've decided to release markItUp because it is implemented on jQuery, and provides a Drupal-esque implementation of "sets" (of buttons and tags) and "skins" (editor themes), and because the keyboard shortcuts in markItUp are very nice indeed.)