From Tim Johnston Development:
Free unit converter made with an engineer in mind. The user interface uses mouse positioning to select the units to convert, but clicking will lock in the selection if desired. The user can adjust the order of the properties by dragging, the foreground and background colors, the font size and the unit separator for the more complex units. There are conversion factors for Area, Density, Energy, Enthalpy, Flow, Force, Heat Capacity, Heat Transfer Coefficient, Length, Power, Pressure, Surface Tension, Temperature, Thermal Conductivity, Time, Velocity, Viscosity, Volume, Weight and it even allows for mole to mass and mass to mole conversions where appropriate.