Desmos is a free online graphing calculator that allows students to graph functions, plot data, evaluate equations, and more. This site has hundreds of digital activities covering grades 6-12, and has the option for teachers to create their own activities as well. Desmos is available to download in the app store for easy access.
Khan Academy is a free online tool that is available for students, teachers, and parents. This site offers short video lessons and practice exercises for many different subjects, including mathematics. Khan Academy covers material from kindergarten to college, including: Trigonometry, Calculus 1 and 2, Linear Algebra, and Differential Equations.
Wolfram Mathematica is an online tool that has a variety of uses. This site covers areas of number theory, algebraic manipulation, mathematical computations, data analysis, graph computations, and much more. Anyone can create an account through the website and receive a 15 day free trail, which includes a download of mathematica and access to mathematica online.