Graphs solutions of diffusion equations over time. Interface could be smoother. You have to click at different points on the timeline to get things graphed - animation would be helpful. The unique feature is that rather than entering arbitrary diffusion coefficients the applets are set to work with selected real-world examples, i.e. diffusion of sugar in water. Multiple applets on one page with explanations. While the site is in Spanish, Google Translate makes it easy to follow in English.