Ever use the "Take a penny, Leave a penny" tray at a the Quikie-mart? This guy at DigitalPoint.com has come up with a type of "Take an ad, Leave an ad" for websites. You put a generic ad on your website that changes dynamically, and can be seen by search engines, and your ads will appear on other sites. It's a great way to promote your site and might even get you higher in some search engine rankings.
The product is called the
Co-Op Ad Network, it's free and growing. Check it out if you like free advertising.