Which body guarantees bank deposits in the US?

The Federal Deposit Insurance Corporation, or FDIC. They ensure up to $250,000 per depositor per institution until the end of 2013. However, recently their reserves have fallen below mandated minimums set by congress because there have been so many bank failures. The FDIC is only required to have about 3.5% of total deposits available to insure losses, because it is highly unlikely that all banks in the country will go broke all at the same time.