When I recently visted Philadelphia I learned that Benjamin Franklin was made the first United States Postmaster in 1775. His post office was on Market Street and is still standing today and is still used as a post office and museum of all things postal.
Did you know that the original postmark was a handwritten signature by Franklin on each letter? This office still uses a copy of his signature as their postmark.
Another interesting fact that I learned while there, is that members of Congress and the Senate have special postmark priviledges. Their postmark is also their signature! I hadn't remembered this fact until I recieved a letter from my congressman inviting me to an upcoming town hall meeting. Sure enough, there where you would normally find the postmark canceling the stamp was his signature. That is a pretty cool perk, wish I could do that!