The Home Affordable Refinance Program, also known as HARP, is a federal program of the United States, set up by the Federal Housing Finance Agency in March 2009 to help underwater and near-underwater homeowners refinance their mortgages.
You may be eligible for HARP if you meet all of the following criteria:
The mortgage must be owned or guaranteed by Freddie Mac or Fannie Mae. The mortgage must have been sold to Fannie Mae or Freddie Mac on or before May 31, 2009. The mortgage cannot have been refinanced under HARP previously unless it is a Fannie Mae loan that was refinanced under HARP from March-May, 2009. The current loan-to-value (LTV) ratio must be greater than 80%. The borrower must be current on the mortgage at the time of the refinance, with a good payment history in the past 12 months.
Source:  http://www.makinghomeaffordable.gov/programs/lower-rates/Pages/harp.aspx