University of Colorado
University of Colorado: Ph Et Interactive Simulations: Lasers
Learn about absorption and emission by creating a laser. Pump the chamber with a photon beam, and manipulate the energy states of the laser's atoms to control its output.
University of Oxford (UK)
University of Oxford: Easy Laser Physics
This is a site which explains the inner workings of Lasers. The site is detailed in its explanations but is easily understood at the junior and senior high school level.