In an oddly circular arrangement: the government pays the Federal Reserve interest and the Federal Reserve pays it back to the government.
At present, the Federal Reserve's balance sheet has swelled to more than $2.9 trillion and will reach new highs in 2013 as the Fed purchases $85 billion a month in Treasury and mortgage-backed bonds. In turn, the growth of these holdings causes the central bank to remit record amounts to the U.S. Treasury, $89 billion in 2012, the highest ever.