Apertium
Kalker - a modern calculator
Kalker is a calculator that supports user-defined variables, functions, ambiguous syntax, differentiation and integration. It runs on Windows, macOS, Linux, Android, and in web browsers (with WebAssembly).
Skychart
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Endless Sky
SpeedCrunch - BitBucket
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.