Desmos is an app and website that has free online graphing, scientific, and matrix calculators.
It also has tools for geometry demonstrations and making activities for a class.
PROS
It's free. Graphing calculators can be very expensive and this gives students easy access wherever they are.
It's easy to use.
It's sometimes easier to use than a traditional graphing calculator, especially when graphing things like conic sections.
CONS
It doens't help students develop computational skills.
Some of the activity maker functions lack basic components like a place to put questions titles when there's more than one question on a page.
USE IN MATH CLASS
Students can use the basic, scientific, graphing, and matrix calculators.
Desmos can be used in class to visualize things in geometry with the geometry tool and some of their pre-made construction, like this one showing the orthocenter of triangles.
It can also be used to make activities with graphs and questions for students.