This is pretty cool, Google is beta testing the functionality to allow Web visitors to change the ads served up by Google AdSense. For example, you see an ad and it reads "Change to Ads About:" with a listing of related ad topics. When you click on a different topic, it then allows you to "Enter a different topic" and search on it. Here is an example site, look at the left hand skyscraper ads, scroll down to under the Google Ad and then click on that "Change to Ads About" option. You will see what I am talking about, if not, here is an image that will clarify.
Forum discussion at DigitalPoint.