Overnight, it's almost free.

"The Federal Reserve entered a new era on Tuesday, lowering its benchmark interest rate virtually to zero and declaring that it would now fight the recession by pumping out vast amounts of money to businesses and consumers through an expanding array of new lending programs.
Going further than expected, the central bank cut its target for the overnight federal funds rate to a range of zero to 0.25 percent and brought the United States to the zero-rate policies that Japan used for years in its own fight against deflation.
Though important as a historic milestone, the move to an interest rate of zero from 1 percent is largely symbolic. The funds rate, which affects what banks charge for lending their reserves to each other, had already fallen to nearly zero in recent days because banks have been so reluctant to do business."

NY Times: Fet Cuts Key Rate to a Record Low, 16. Dezember 2008

"The Federal Reserve will employ all available tools to promote the resumption of sustainable economic growth and to preserve price stability," the Fed said in a statement. It added that it expected interest rates to remain "exceptionally" low for some time, a subtle commitment to the current policy that could help bring down longer-term interest rates.
In normal times, lower rates reduce the cost of borrowing for households, businesses and financial institutions, which spurs borrowing and economic activity. Those effects are being muted now, however, because many businesses and households are weighed down by heavy debts."

Wall Street Journal: Fed Cuts Rates Near Zero to Battle Slump, 17. Dezember 2008