Since ideas are a dime a dozen: Glassdoor for Apartment Rentals: Tenants give their address and rent amount per month in exchange for access to everyone else's data. Know what other people are actually renting for, not landlord asking rates. Why is there no data on what your neighbor just signed his lease for?
Angelist for Real Estate
Imgur for Excel
Nextdoor for Office Buildings
I got loads more...got to be something worth making in there... :)