Tightened lending standards are leaving builders and real-estate agents scrambling for new ways to move cash-strapped buyers into homes. One increasingly popular option: an obscure home-loan program offered by the U.S. Department of Agriculture.

Created in 1991 as a way to boost homeownership in rural areas, the program is being tapped by home buyers in overbuilt exurbs who are attracted to the no-money-down terms.

When...