This widget allows visitors to easily diff posts against their earlier revisions, displaying diffs of HTML inline.
It should be plug and play: Just add the widget to a sidebar that will appear on a page or post. It will detect everything it needs. If you have more complex needs, the code is heavily documented and includes many filters to modify behavior. If the widget's design doesn't suit your purposes, its class is easily extendable, with methods for generating all of the necessary HTML.