"Perennial Weblog Tools Collection community favorite Who Sees Ads can be used to show ads only to certain visitors – such as people who have arrived on your site from a search engine. It’s a useful way to improve your click-through-ratio, and please your regular readers by hiding ads from them.
This is a quick introduction to using the plugin. Showing ads only to search engine visitors is extremely simple – Who Sees Ads can do much, much more. Check out the links at the end of this article for more information."