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

SpeQ Mathematics - SpeQ is a small, extensive mathematics program with a simple, intuitive interface.

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

Qalculate! - Qalculate! is a multiplatform multi-purpose desktop calculator.

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.

NumWorks - The graphing calculator that makes learning math easier.