I have just come across a beautiful, free, easy and intuitive to use online graphing calculator from desmos.com.
It instantly plots functions, lets you create tables, add sliders, animate your graphs, and more, making it a valuable resource for teachers and students. The key is learning by doing. The calculator can instantly plot any equation, from lines and parabolas up through derivatives and Fourier series. Sliders make it easy to demonstrate function transformations. One can change parameters and instantly see the changes on the graphs making experimentation easy, quick and without the hassle of doing a lot of manual calculations to see what happens.
Graphing: Plot polar, cartesian, or parametric graphs. There’s no limit to how many expressions you can graph at one time – and you don’t even need to enter expressions in y= form!
Sliders: Adjust values interactively to build intuition, or animate any parameter to visualize its effect on the graph
Tables: Input and plot data, or create an input-output table for any function
Statistics: Find best-fit lines, parabolas, and more.
Zooming: Scale the axes independently or at the same time with the pinch of two fingers, or edit the window size manually to get the perfect window.
Points of Interest: Touch a curve to show maximums, minimums, and points of intersection. Tap the gray points of interest to see their coordinates. Hold and drag along a curve to see the coordinates change under your finger.
Scientific Calculator: Type in an equation you want to solve and Desmos will show you the answer. It can handle square roots, logs, absolute value, and more.
Inequalities: Plot Cartesian and polar inequalities.
This is one I will be using with my students.