This tool caches Wikipedia pages locally in your profile, when pages change it notifies you and you can opt to send the pages to the WikiDiff server to perform a visual diff that unlike other diffs preserves the formatting of the webpage.
This version 0.1 is a Beta version.
If anything is broken that is reproducible, I'd like to know about it.
How to use it:
The main way to test immediately it is to go to any wikipedia page (pages are cached specially for the extension), then go to the history tab and load an old revision (by clicking on the date link), you should see a yellow bar open up asking you to compare the differences. You'll notice that you are taken to my website.
However it's main usefulness, is that when you return to a page you have visited previously it will alert you as well.