AffordableHousingOnline.com provides affordable rental housing market data on every city and county in the U.S. For example, here is the affordable rental housing market page for Washington, DC: http://affordablehousingonline.com/housing-search/District-of-Columbia/Washington/
AHO's own proprietary database of Federally assisted housing is mashed up with Census place data to provide an innovative look into rental housing trends.
The site is used by renters searching for affordable housing as well as advocates who wish to help those in need. The layer of Census data from the Census API provides data snippets such as the percentage of the population that is rent overburdened, the total economic impact rental housing has on a place and the rental (or homeowner) nature of a place.
The web site receives more than 2 million visits per year.