“Citizens who wish to make a general donation to the U.S. government may send contributions to a specific account called ‘Gifts to the United States.’ This account was established in 1843 to accept gifts, such as bequests, from individuals wishing to express their patriotism to the United States. Money deposited into this account is for general use by the federal government and can be available for budget needs. These contributions are considered an unconditional gift to the government. Financial gifts can be made by check or money order payable to the United States Treasury and mailed to the address below.”
Thanks to Mark Perry for the pointer.