[tabiv]

This is my favorite online graphing calculator. It's been around for a while. It got me through pre-calculus.

[mhh__]

I use it for (some) quant finance work. It's not a toy, shockingly versatile.

[jasinjames]

I recall using desmos to render solutions to constraints in my physics classes in college. My favorite was plugging in the equation for the normal modes of a driven, oscilating square sheet and watching the vibration pattern pop right onscreen.