Desmos is a fantastic free online graphic calculator. It is super easy to use, also. It starts drawing the graph while you're still typing!
Users can save graphs... see for example this one: a function, its derivative, and tangent drawn to a point. You can change the function as you wish. I changed it a little bit, and this is what I got:
Here's another example: Riemann sums that someone created. Again, you can change the function and other parameters.
The graphs can also be printed, emailed, and embedded on web pages.There's lots you can do: parameters (with sliders), inequalities, piecewise functions... In fact, many users have used it as a drawing platform by creating a bunch of piecewise functions, and so they come up with pictures like this one: