errProof - The website has a lot of useful and unique tools.
SpeedCrunch - SpeedCrunch. SpeedCrunch is a high-precision scientific calculator featuring a fast, keyboard-driven user interface. It is free and open-source software, licensed under the GPL. Download Documentation Donate .
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.
Qalculate! - Qalculate! is a multiplatform multi-purpose desktop calculator.
Insect - High precision scientific calculator with full support for physical units.
Numi App - Numi is a beautiful text calculator for Mac.