A cutting-edge science experiment left behind in the Sea of Tranquility by Apollo 11 astronauts is still running today...

a 2-foot wide panel studded with 100 mirrors pointing at Earth: the "lunar laser ranging retroreflector array." Apollo 11 astronauts Buzz Aldrin and Neil Armstrong put it there on July 21, 1969, about an hour before the end of their final moonwalk. Thirty-five years later, it's the only Apollo science experiment still running

(from Science @ NASA).