SkinCalc, which is completely free, features standard and advanced calculator functionality, a spreadsheet interface, and skins.

Skins make it possible to change the layout and functionality of the software, with effects that include non-rectangular shapes, user-configurable transparency and alpha channel translucency.

The current version of the calculator engine was designed to be a fully skinnable application which can completely replace the Windows system calculator operating in Standard mode, with the addition of certain advanced features not available in the system calculator.

More than 100 skins are available from the sites listed at skincalc.com, which also has instructions to create your own skins.