it seems we can search google's ad database, for example if we do any query in this URL;

www.google.com/sponsoredlinks it shows ads which google shows for adwords are adsense, seems like a good feature to know who are the advertisers for a particular keyword phrase and how many ads are displayed for a given keyword,

                                Earthlink Netscape Netvouz RawSugar Shadows Sphinn StumbleUpon Yahoo MyWeb