We also had boat letterhead printed, with USCG number and other official looking stuff. Some countries custom's people really like official looking things - like when we had to return to a country after checking out and they needed a "letter from the Captain" explaining why to put in their file.
We also have some printed with all the required boat/crew info in French and Spanish.
Another thing I have seen other boats have is a cool looking official stamp. Officials all over the world seem to love stamps.