Zillow a site We love, has begun to develop an open API to share data on 67 Million Homes.
Zillow I think has one of the coolest interfaces for viewing home values, and with an open API web designers and online real estate marketers  have the ability to incorporate the data and cool graphics into their blog or website making for a cool experience for both buyers and sellers researching online. (78% of consumers start their search online)
Here are a couple examples of it in Use:

At Yahoo:

Real Estate Home Value

Zillow result

Zillow Example Home Value