Planck's constant is one of the fundamental constants in the universe. For example, it defines the relationship between the energy of, say, a photon and its frequency. It's widely used in the mathematics of Quantum Mechanics. Recently, using just a Lego kit and some other equipment, including a laser, Leon Chao at the National Institute of Standards and Technology in Gaithersburg, Maryland, showed how to make this measurement. This is for real, and it works because the experimental technique simply requires a well crafted balance. It's all written up in the M.I.T. Technology Review. You could actually do the experiment yourself because the costs of parts is only about US$500. Very cool.
Check It Out: How to Use a Lego Kit to Measure Planck's Constant