Check out Add-Art: http://add-art.org/ - "a free FireFox add-on which replaces advertising on websites with curated art images."

Here's a chrome extension that lets you toggle kittenification on or off. (It's not blocking ads yet-- I'll get that done eventually) https://chrome.google.com/webstore/detail/fgppoekgokpnoigpci... code: https://github.com/gk777/Kittens-Everywhere

Done. http://stanford.edu/~grantm/cats.html

It actually turns _all_ images into kittens, but I think it's an improvement. :-)