Hi,
To the OP, what you are looking for is MLS software and it is already here. There are several companies who provide this service and they guard the data very well. There are companies that provide dedicated software, hardware and database storage to MLS boards who enter the data in from real estate agents or their employees. The data is not just freely available for anyone.
Not sure how Realtor.com and others get to the data but they are probably through contracts with the different MLS boards who contract with the companies to provide the MLS solutions.
Programmatic access to the data is through a standard called RETS and here is a link to describe it.
http://en.wikipedia.org/wiki/Real_Estate_Transaction_Standard
Here is a better web site for RETS info:
http://www.realtor.org/field-guides/field-guide-to-real-estate-transaction-standards-rets
My Google search turned up this tool:
http://www.retsconnector.com/. Here is the key piece of information from CoreLogic's page for their RETSConnector:
RETS Connector is provided as a service to our ML customers and their members to facilitate secure, standardized, programmatic access to their ML databases.
Notice they say "our ML customers and their members" which means it is not open to the public. It probably requires log on credentials just like any other MLS system.
Before anyone says anything, this is my day job, working on MLS software (but not RETS) for a global company. Our company employes its own RETS team who site about 3 cubes away from me. I get involved in their discussions and hear other discussions from them. It can be interesting at times.
Not sure how Winnow will work, especially when a region/city is served by another company through contracts. That could be interesting. Still, it will require $$$ to sign up.
[UPDATE] The requirements for RETS Connector seem to indicate the need for a Windows based system.
