A free online graphing calculator and math tools.
Desmos offers a browser-based graphing calculator plus scientific, geometry and other math tools, and classroom activities. Many tools are free to use (as of writing).
The calculators are free to use.
Yes, apps are available for iOS and Android.
Information and links are accurate as of the time of writing and may change. Always verify on the official site. This is an independent guide, not affiliated with the listed services.