If you like the simulations @ http://phet.colorado.edu/ you'll love Molecular Workbench!
What's MW? Molecular Workbench (MW) provides visual, interactive computational experiments for teaching and learning science. Hundreds of Free Simulations Explore physics, chemistry, biology, biotechnology, and nanotechnology with interactive simulations. Embedded Assessments Real-time reports provide a complete view of student learning progression that teachers can track.
