When my wife and I were looking at buying an investment property, I built a little program to use Zillow's API to get tons of data for a particular area, make some calculations, and then sort the data in order of most potential profit as a rental (based on their estimated rental rates, which are not always perfect of course). Then I added some filters so I could find a property that met our other criteria.

It worked really great, and we never ended up buying a property, but I couldn't help thinking this is a premium feature that Zillow could easily offer and charge for. If anybody at Zillow is interested, feel free to reach out.