Desmos is a free graphing and teaching tool for math available on the web as well as on iOS and Android. In addition to plotting equations, classroom activities are available to help students learn about a variety of math concepts. For example, students can learn how to transform periodic functions by trying to slide marbles through points on a graph. Or they can plug in their own equation and see what kind of graph pops up; sliders allow students to adjust values and see what happens. Users can also click directly on the graph to find the coordinates of points of intersection, maxima, and minima.