Bantr is a Firefox extension that replaces some 300x250 banners with tweets from your friends. The site says the tweets are matched to the context of the page whose ads it's blocking. I didn't really notice, but I am not following thousands of people, so maybe that's why. It doesn't replace every single unit, just one, and not on all sites, although it does on AdLab.
Wouldn't it be a fun ironic twist if the tweets Bantr displayed were affiliate ad tweets?
Or you can replace banners with art.