Fannie Mae  and Freddie Mac  are government sponsored enterprises (GSE). Yet they have shareholders and are profit driven. They play a critical role in the stability of the US mortgage market (and housing) by promoting liquidity, helping mortgage rates and availability consistent throughout the country.
One of the things that made them have a competitive advantage over others was their inferred backing by the federal government.
In the New Yorker this week, James Surowiecki writes in his column Sponsoring Recklessness 
The two companies have long been required to tell investors that their securities are not guaranteed by the federal government. But in the financial markets everyone has always assumed that this demurral was just window-dressing, and everyone, it turns out, was right. Last week, when fears of a possible collapse of the two companies threatened to spark a major financial crisis, the Treasury Department and the Federal Reserve quickly came up with a rescue package. What had been an implicit guarantee became an explicit one
Fannie was privatized in 1968 so president Johnson could move the debt off the federal books to help sell the Vietnam War budget, not to help the mortgage market.
Help to the consumer in terms of their impact on keeping low mortgage rates may be exagerated.
A paper by the economist Wayne Passmore , of the Federal Reserve, suggests that in fact Fannie and Freddie have only a small effect on the interest rates that homeowners pay, saving them less than one-tenth of a percentage point.
The GSE self-preservation mechanism has been aggressive lobbying using former high placed government officials, very effective in enabling them to grow to $5 trillion in mortgage debt. A blip on the radar could cause more damage than Congress is able to burden the taxpayers with.
More than $10 billion in losses in the past two quarters, the GSEs (and FHA) are looking for more money to capitalize  to help bailout the housing market at Congress’ urging.
Holden Lewis over at Bankrate wrote a great post on this last week called The GSEs and moral hazard .
Daniel Gross, my friend over at Slate and Newsweek, makes a better argument for the help GSEs provide to the taxpayer/homeowner suggesting that a bailout of the GSEs would actually be a bargain .
I guess I have a hard time accepting that anything the federal government would do would be a bargain and the long term concept of nationalization of the GSEs would be cost effective, but hey, I don’t have to refinance my mortgage.