Zillow offers Real Estate and Mortgage data for individual sites and now-- QiSoftware offers API and software support..

Zillow provides a number of options for those wanting to add Real Estate related information to their site, including:

The following is a chart example of the type of information that is offered under the Home Valuation API. The chart shows the property values for a specific property over the course of a year.

In researching selling points for Real Estate related information-- I searched on "What are home buyers looking for online?". I found the following response on the "realestate forum":

In the near future, I will probably provide interactive examples for uses of this powerful API via the QiSoftware Products & Services catalog. In the meantime, if you would like to investigate using this type of data on your site-- you can contact me here.