Desmos in a FREE online graphing calculator that can be used in many different ways. Graphs can be saved and shared allowing teachers and students to easily access and share work. There is also a teacher desmos where teachers can set up lessons and example problems that the students can complete independently.
GeoGebra is another FREE online graphing calculator. It is very similar to Desmos, however, GeoGebra can also be used as an online geometry tool as well as a graphing calculator.