Many useful graphs and a lookup the debt by date function.
Also under the FAQ you can find this helpful info :
How do you make a contribution to reduce the debt? Make your check payable to the Bureau of the Public Debt, and in the memo section, notate that it is a Gift to reduce the Debt Held by the Public. Mail your check to:
Attn Dept G Bureau Of the Public Debt P. O. Box 2188 Parkersburg, WV 26106-2188