The Desmos graphing calculator is outstanding, easy to use for both graph creating and sharing and entirely free! I use it in the classroom all the time. I have written several posts on this which can all be found below. These posts feature many examples.
Desmos Calculator Examples (a series of illustrations to help students with a coordinate geometry assignment).
Desmos features Points of interest and Parametric Equations
Desmos Graphing Calculator (trace, sliders, modulus function and more)
…and my very first post in June 2011 when I discovered Desmos!