Georgia State University
Georgia State University: Hyper Physics: Interference
An indexing page for a physics department site about the wave properties of light that lead to interference.
Khan Academy
Khan Academy: Diffraction and Constructive and Destructive Interference
Article goes into detail about what exactly is interference.
Boston University
Bu: Optics Demonstrations: Interference
Several short descriptions and graphics of demonstrations which illustrate principles of interference. Suitable as a supplement for a teacher lesson plan or as an idea-generator for a student project.