https://www.prodigygame.com/ - Content-based with a video game feel
https://www.kenkenpuzzle.com/# - order of operation puzzles (also a phone app)
https://cignition.com/ - Real-world scenarios for using math skills
https://www.polyup.com/ - Function machine for a variety of levels
https://play.centerforgamescience.org/refraction/site/ - Fraction game with lasers & space ships
https://www.dragonboxapp.com/ - Teaches algebra through visuals ($)
https://ca.bigideasmath.com/students/?free=1 - Big Math Ideas (Student Resources) provides parents and students with links to basic and review math skills as well as a "game closet" that keep students engaged.