A bismuth-based superconducting disk sits on a copper base and is immersed in liquid nitrogen. Two small, very strong magnets are placed on top of the superconductor. The magnets levitate due to the expelled magnetic field from the superconductor. Superconductors have the unique property of providing zero resistance for electrical currents when cooled to low enough temperatures. Thus, a current induced in a superconductor will last for an extraordinarily long time. This property also leads to the fact that superconductors will expel all magnetic fields, a phenomenon called the Meissner Effect. As a result of this effect, a magnet approaching a superconductor will "see" an opposite magnet of the same size and field strength as itself, which causes it to levitate.
Other demonstrations from MIT