Now this is cool. As you may know, 3 of the Apollo missions and one of the Soviet Lunokhod rovers left laser retroreflector experiments with arrays of corner reflectors that would bounce a laser beam straight back from where it came, allowing Earthbased telescopes to measure the distance to the retroreflectors. Up until now, only the McDonald Observatory in Texas and a couple other experiments in other countries have been used to make such measurements and with a precision of something like a few centimeters.
This relatively new program that went into operation in 2005 uses the 3.5 meter telescope at Apache Point in New Mexico is called, somewhat appropriately, APOLLO. Incredibly, they've made big improvements over previous experiments such that they are achieving accuracies of about a millimeter in their distance measurements to the Moon! They are measuring times of photon returns to a few picoseconds! Their equipment receives photon returns substantially more efficiently than previous experiments.
With this kind of capability they will be able to test Einsteins theory of General Relativity as well as the precision of the inverse square law of gravity itself. It can measure the constancy of the Gravitational constant G, not to mention measuring the exact motion of the Moon as well as of their own observatory over time which includes the affects of continental drift, all to higher precision than has ever been done before.