Quick Number Base Converter 

Quick Number Base Converter is a small utility application that sits in the windows system tray. When you need to converter between decimal, hex, binary, etc -- just click the icon and the converter pops up. This software is freeware.
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Calc98 

Scientific engineering statistical and financial calculator, with conversions, constants and physical property data, a built-in periodic table of the elements, stopwatch, number base conversions, vectors, matrices and complex numbers.
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