AddArt is a Firefox add-on developed by the American artist Steve Lambert.
The concept is inspired by the popular Adblock add-on which removes ads from web pages. The main difference between the two is that AddArt will not just block ads, it will replace them with new images - images created by artists. The more ads you surf, the more art you'll get.
The idea is to run AddArt like an art gallery with different curators responsible for organizing the shows.
AddArt is currently in prototype-mode but hopefully it will be functional soon.