University of Colorado
University of Colorado: Physics 2000: Lasers
If you want to know how atoms and photons work together to make lasers this site has great explanations. There are interactive applets to help you really understand the concepts!
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.