I use Desmos all the time when teaching. It's accessible to students with no programming knowledge, and the killer feature is that I can just give students a link so they can interact with a demo I've made.

For example:

Quadratic through 3 points (drag the points around): https://www.desmos.com/calculator/tf1f80zgug

Pythagoras's Theorem (drag the sliders): https://www.desmos.com/calculator/dbjxbeuzk7

Why shearing preserves area (drag the slider): https://www.desmos.com/calculator/1vykaqf7je