Desmos, the Free Online Graphing Calculator
Desmos combines the typical functionality of a graphing calculator with dynamic representation capabilities. The result is an intuitive and powerful tool for demonstrating and exploring mathematics.
Desmos is one of the best things to happen to math in years! It has quickly become an essential part of my math classroom.
Check out the software itself at Desmos.com.
And you can see some of the ways I use Desmos in and out of the classroom here, including resources from my Introduction to Desmos workshops.