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.
Simulations are available for Physics, Chemistry, Earth Science, Biology,and Math. Teachers can use the simulations as part of a lecture, blended lesson, or as extra practice for the students. When students practice with the simulation they get immediate feedback and can continue to attempt each problem until they get it correct.