Math tools:

Geogebra:
https://www.geogebra.org/3d
Can graph functions of 2 variables in 3D

https://www.geogebra.org/geometry
Can draw geometry diagrams with mouse or with command line. Useful for certain olympiad geometry settings.

Euclidea.xyz – a geometry construction game
https://www.euclidea.xyz/
Exercises in ruler and straightedge constructions in geometry
(also a phone app)

Wolfram Alpha
Can do lots of things, including be a calculator