PhET Interactive Simulations
Founded in 2002 by Nobel Laureate Carl Wieman, the PhET Interactive Simulations project at the University of Colorado Boulder creates free interactive math and science simulations. PhET sims are based on extensive education research and engage students through an intuitive, game-like environment where students learn through exploration and discovery.
These simulations are free for instructors to use, distribute, and modify - even in commercial ways. Attribution must be given to the original source, however.