From the official google blog:

Homes, not just homepages
More than ever, home buyers are starting their search online, and we want to make it easier for every one of you to find the home of your dreams. Searching on Google for Seattle real estate or homes for sale in San Diego prompts you to enter a location and choose whether you want to buy or rent.* After clicking "Go," you can see the individual homes that Google has indexed, provided by our partners and culled from the web. When you want more information on a particular home, you can click straight through to the source of the listing-no detail pages or sign-up forms get in the way. And when Google gets the same listing from multiple sources, we show links to all the data providers and websites, ranked according to many factors including, but not limited to, the quality and comprehensiveness of the data.

If you're a real estate professional, making your listings searchable on Google is simple and completely free. We don't charge for photos or offer "featured listings." We believe that buyers just want to see the home that fits them best and that providers shouldn't have to pay to show it to them. We don't sell houses, deal with agents' compensation, or charge for leads. read more

No charge for leads, additional photos "featured homes" click straight through to the source.  Unlike the Yahoo Prudential where agents are paying big referral fees,  the Keller Williams Listing Services puts listings on Google and Trulia with tons of photos FOR FREE.  Google's model it to make money by advertising and having content, while Yahoo, looks to extort the agents in the traditional lead generation model.  Thank you Google!

 Yahoo real estate vs. Google Real Estate  Read an interesting article about Yahoo, Purdential, Google, Keller Williams Realty