SkinCalc

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