Convert is a portable free tool which can convert measurements of mass, distance, area, volume, force, speed and many others from one unit to another.
The program has a very simple and straightforward interface. There are tabs for every measurement type; clicking any of these lists the possible input and output units; and after you've made your choice, entering something in the Input box will instantly display the converted value.
This all looks a little basic, but Convert does have some small but welcome bonuses. Like the ability to add multiple "Custom" conversions of your own, with a source unit name, a destination, and the conversion factor.
If you've no interest in converting "light", or you're not exactly sure what "micromoles" are, then you'll appreciate Convert's ability to hide some tabs and reorder the rest.
And if you'd like to run the program anywhere then the good news is it's compatible with everything from Windows 95 upwards.