DiffWidget.js
DiffWidget is a jQuery plugin for create slider/diff viewer as at jpegmini.com. Clone it and run index.html for try.
Usage
$('#ex3').diffWidget( { 'width': '500px', 'height': '200px', 'top': 'images/forest.jpg', 'bottom': 'images/forest_invert.jpg', 'position': '75%' })
Other
Images from flickr.com