Short Answer: Adsense for Realestate.
The following MSNBC article says there are 6,760,000 homes sold in the last year for an average price of $223,000. The average commission on a sale of a home is 5% or $11,500 per home. 6,760,000 homes * 11,150 commission = $75 billion in commission
The affiliate marketer in me says that realtor.com should create adsense like product for realestate. If someone clicks on an ad and as a result places a call, or emails an agent all that can be tracked and recorded. If a sale takes place as a result of the click the referring website should get a cut of this commission.
Newspapers are the companies best positioned to take a huge chunk of this market, other companies like zillow.com would be worth billions overnight. Given that I have hundreds of thousands of unique visitors a day It does not seem outlandish that a site like mine could sell 2-5 homes a day which would by far out monitize what adsense pays, and my site has nothing to do with homes.
So come on Google, Yahoo, Realtor.com etc Who ever builds this first send me an email I want to be in the beta.