Khan Academy: https://www.khanacademy.org/
XtraMath: https://xtramath.org/#/home/index
Freckle: https://www.freckle.com/math/
Illustrative Mathematics: https://tasks.illustrativemathematics.org/content-standards (Use if you know what Common Core Math Standard you are currently working on in class.)
Prodigy: https://www.prodigygame.com/
Math Playground: https://www.mathplayground.com/games.html