Ellen Holderman has an excellent post about “24 Apps, Games, and Websites Teachers are Using in STEAM Classrooms” that are mostly free. (STEAM stands for Science, Technology, Engineering, Arts, and Mathematics).
They’re mostly aimed at a younger audience–middle school and below–, but there are things like Khan Academy that’s useful for all ages. Quite a number have to do with introducing coding.
↬ Ms. Lannert.