https://www.desmos.com/calculator
There’s obviously many options for graphing calculators out there, but this one is so simple and well implemented that I just keep coming back to it when I want to explore mathematical and geometric concepts.
For example, here is a saved graph I recently created to explore SamSi’s Heart of the City puzzle:
https://www.desmos.com/calculator/5ok5wcblyy
- danBhentschel