Lasers are an interesting application of quantum mechanics (specifically stimulated emission; laser stands for light amplification by stimulated emission of radiation). I think the idea is that you have a bunch of atoms all in a particular excited state (E.g. first excited state). A photon hits one of the atoms, which knocks it down into the ground state energy level, emitting another photon with a frequency such that it will make up the energy difference between the first excited state and the ground state. Now we have two photons (the photon we sent out to begin with, and the one from stimulated emission). These two photons will both hit atoms in the first excited state, and knock out two more photons, with exactly the same frequency (proportional to the energy difference between first excited state and ground state). This starts a chain reaction creating coherent radiation, which in optics is known as the "amplification effect".