Desmos is an education startup, currently with one major offering – an online graphing calculator. Graph any equation, cartesian or polar coordinates, exponential and trigonometric functions, and multiple equations at the same time. Using MathQuill (a what-you-see-is-what-you-get LaTeX editor for HTML5) and/or a built-in key pad users can easily enter and seamlessly edit any equation.
Users can also create a free account to save graphs and use a projector mode to display on in-class screens.
I am currently trying to rebuild my own mth skills after years of neglect, starting from precalculus on through multivariable calculus. My main tool for this has been Khan Academy, which has a great set of exercises arranged in a knowledge map, but a TERRIBLE calculator interface. I haven’t had a chance to play with the two simultaneously yet, but I strongly suspect that Desmos’s calculator is going to be a hugely useful tool for visualizing and manipulating complex functions.
Additional awesomeness include the explorations, where users can explore graph-able functions ranging from different forms of linear equations, through derivatives; and the graph gallery, where users can peruse other users recently saved graphs.