Qalculate! is a multi-purpose cross-platform desktop calculator. It is small and simple to use but with much power and versatility underneath. Features include customizable functions, units, arbitrary precision, plotting, and a user-friendly interface (GTK+ and CLI).
Qalculate! has long been available for GNU/Linux and Mac OS X, but now it is finally also compatible with Microsoft Windows (although there is a fork by Cheng Sun with Windows compatibility, which has contributed code to this version). This pre-release version is likely not as stable as version 1.0.0 (although it does include some important bug fixes), but you are nevertheless invited to try it, report any issues you find and share your impressions.
Download the installer for Windows here:
qalculate-1.0.99.msi
The installer includes both the GUI and the CLI program, and requires at least Windows Vista or later. All dependencies are included.