http://www.walter-fendt.de/ph14e/huygenspr.htm
While hardly new, I have used this applet upon multiple occasions in two classes I taught this semester, and will again use it in two classes next semester and feel that I should therefore proclaim its utility to anyone teaching E&M, waves, or optics who is establishing connections between the three topics. This animated applet powerfully illustrates how Huygens' principle can be used to explain reflection and refraction (and determine Snell's law) given different speeds of propagation in different materials. I include this applet when transitioning to optics in my E&M course, and for juxtaposing the ray, wave, and QED models of light reflection and refraction to more advanced students. A wonderfully executed animation, translated by Taha Mzoughi.