University of British Columbia
University of British Columbia: Single Slit Diffraction Pattern of Light
Explains single slit diffraction and models how to calculate different characteristics of the light waves.
CK-12 Foundation
Ck 12: Physics Simulation: Marina
[Free Registration/Login Required] Learn about the interference and diffraction patterns that result when a wave passes through one or two thin openings using this interactive simulation. A PDF worksheet and a video tutorial are also...
Khan Academy
Khan Academy: Diffraction and Constructive and Destructive Interference
Article goes into detail about what exactly is interference.
Boston University
Bu: Optics Demonstrations: Diffraction
Short descriptions and graphics of three demonstrations which illustrate the diffraction phenomenon. Suitable as a supplement for a teacher lesson plan or as an idea-generator for a student project.