My idea is a mortgage calculator on steroids. You’d be taken to a well-designed site where, instead of entering an estimated total payment, you’d be asked for a street address. The site would take advantage of the Zillow API to pull the “Zestimated” home value along with the property tax info and other available information. The user would then be presented with the estimated monthly payments for that property, along with a few visual levers you could use to adjust rates/credit rating, term length, loan type, etc… Additional information such as a map showing surrounding property values (again, thanks Zillow!), amortization schedule, potential discounted purchasing options from an online real-estate agent like Redfin, along with other data pulled from real estate web services.
I actually think this is a great opportunity, especially considering how competitive the CPC rates are for mortgage/real estate ads (Scoble puts it at $60 a click, not sure how accurate that is). Plus, with the advent of the new real estate web service APIs that are maturing, it makes for an easy mashup site that could pull in a decent monthly AdSense income without much effort or maintenance.
If I had the time, I might actually code this one up myself. Between work, house hunting, and the time I spend on the one idea I don’t want the Internet to steal, I’m a little maxed out. 🙂