Apollo left mirrors on the Moon. Observatories still fire lasers at them, and the round trip is measured to the millimetre.

Apollo 11, 14 and 15 deployed corner-cube retroreflectors, joined by two Soviet Lunokhod rovers. Fire a laser pulse, wait about 2.5 seconds, and catch the handful of photons that make it back. Fifty years of that data is where the Moon's recession rate comes from, and it's still testing general relativity.