Qalculate! is a multi-purpose desktop calculator for GNU/Linux (and Mac OS).
License: Open Source
Apps available for Linux Xfce
OpalCalc is an interesting combination of a scientific calculator and a spreadsheet, which comes packed with features.
License: Freemium
Apps available for Windows
Qalculate! is a powerful calculator with extensive features suitable for advanced mathematical tasks, while OpalCalc offers a more straightforward and user-friendly approach for basic to intermediate calculations.