Amazon ads

Amazon has a ad lineup that is similar to Adsense in that it automatically matches the ad content to your sites content. For example, if you have a website or blog on books, Amazon's ads will show listings for books that are currently for sale on their website. You earn a portion of the sales revenue, so these ads are similar to a referral program, as you only get paid  when someone makes a purchase. While not the best ad service, it should definitely be considered.