Desmos - A beautiful, innovative, and modern online graphing calculator.

Insect - High precision scientific calculator with full support for physical units.

WolframAlpha - WolframAlpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.

errProof - The website has a lot of useful and unique tools.

GeoGebra - GeoGebra is free and multi-platform dynamic mathematics software for learning and teaching.

Getcalc - getcalc.com - online calculators, formulas & work with steps (step by step calculation) for expressions, statistics, probability, finance, health, geometry, general purpose & engineering calculations along with basic & scientific calculators.