From the Add-Art site:

Add-Art is a free FireFox add-on which replaces advertising on websites with curated art images. The art shows are updated every two weeks and feature contemporary artists and curators.

Spearheaded by Anti-Advertiser Steve Lambert, this open source project is being developed by Eyebeam. It works with AdBlockPlus, a Firefox extension that replaces online ads with blank space. This extension turns that blank space into art. Shows rotate every two weeks.