Signed, Richard Nixon

Left behind on the moon:

This commemorative plaque, attached to the leg of the Lunar Module (LM), Eagle, is engraved with the following words: “Here men from the planet Earth first set foot upon the Moon July, 1969 A.D. We came in peace for all of mankind.”
It bears the signatures of the Apollo 11 astronauts Neil A. Armstrong, commander; Michael Collins, Command Module (CM) pilot; and Edwin E. Aldrin, Jr., Lunar Module (LM) pilot along with the signature of the U.S. President Richard M. Nixon.

[via nasa images]