TechSheet 

TechSheet a technical spreadsheet that allows for calculations that incorporate units of measure with associated values in each cell. Numerical manipulations result in consistent units of measure and automated unit conversions.
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SAM 

SAM is a collection of useful utilities that can help you in day to day life. SAM is easily accesibly via the system tray or using keyboard shortcuts. The modules include a Reminders manager, Notes, Tasks, Unit conversion, World time clocks and more.
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Archimedes Grapher Professional 

Archimedes Grapher is a kind of powerful and convenient tool software which supports functional graph plotting, numerical calculation, unit conversion, and slide show.
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Uconeer 

Freeware. 397 units in 46 categories. Includes engineering specific categories like enthalpy, heat capacity, heat transfer coefficient, torque and viscosity. Work with feet and inches. Formatting and clipboard features. Excludes archaic units.
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Converter Pro 

Converter Pro comes with a database that contains the most important conversion factors to convert length, area, volume, mass, force, pressure, density, energy, power, temperature, dynamic viscosity, kinematic viscosity and time units.
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