And the second biggest holder of US debt is...the EU.

As of May 8, 2007, the US Treasury says that the US owes

US $8,828,073,000,000 (rounded to the nearest million). That's eight trillion, eight hundred and twenty-eight billion, seventy-three dollars.

On January 21, 2000, the US debt was US $ 5,711,818,000,000.

Both figures are from the US Treasury Department's daily summary and represent the TOTAL debt (as far as I can tell -- I'm a librarian, not an accountant).