Desmos - A beautiful, innovative, and modern online graphing calculator.
Jim - An overview of Jim capabilities
WolframAlpha - WolframAlpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
Insect - High precision scientific calculator with full support for physical units.
GeoGebra - GeoGebra is free and multi-platform dynamic mathematics software for learning and teaching.
errProof - The website has a lot of useful and unique tools.