Q: To whom do we owe all this money? Who owns the Debt?
A: Here is a pie chart showing the makeup, or ownership, of the National Debt as of December 1998.
As you can see, the largest slice of the pie, over 40%, is owed to the Federal Reserve, the central bank of the United States, and to other government accounts. BTW, The Fed is actually quasi-public (part private, part government) so calling it "part of the government" is not strictly true. You can find out more about The Fed by reading Wikipedia's excellent article.
The remaining 60% of the Debt is privately held by individuals, corporations, states, and foreign governments. As of November 2007, Japan ($580 billion), China ($390 billon) and the United Kingdom ($320 bilion) are the biggest foreign holders of our Debt.
Q: When did the Debt pass the $8 trillion mark?
A: On October 18th 2005, the Outstanding Public Debt rose to $8,003,897,406,911.24 -- the first time it had risen above $8 trillion.
Q: When did the Debt pass the $7 trillion mark?
A: On January 15th 2004, the Outstanding Public Debt jumped $13 billion to $7,001,852,607,623.35. This was the first time in history the U.S. National Debt surpassed the $7 trillion mark and came less than two years after the Debt first passed $6 trillion.
As a comparison, the National Debt took over six years to rise from $5 trillion to $6 trillion.