Charlotte's Revit User Group comes the Unit Converter add-in.
"Converts the most popular units of distance, volume, time, speed, mass, power, density, pressure and energy and more. It contains over 4,500 combinations and includes options to e-mail a conversion as well as suggest units and copy the output to the clipboard."
This add-in may be downloaded from the Autodesk Exchange Apps website. Cost is USD 2.99.