Web page diff

[Warren Block]

Does such a thing as a "web diff" exist?  A graphical diff, somehow 
showing two pages side-by-side and highlighting areas that have changed.  That would 
make checking content changes much easier, no diff(1) output or markup 
to distract from the content.

So far the only thing that I've found is a plugin for IE: 
http://www.ghacks.net/2010/12/12/diff-ie-highlight-webpage-changes-in-internet-explorer/

Untested because, well, it's for IE.