Slick MooTools WYSIWYG Editor: MooEditable
MooEditable is a MooTools plugin that instantly converts a textarea field into a WYSIWYG editor.
It has a slick interface & comes with only the frequently used items like text formatting, lists, inserting images/links & undo/redo.
The plugin can also switch between design & HTML modes.
If you'd like to improve it further or customize to your needs, It has a very detailed documentation.
Compatibility: All Major Browsers