ESBUnitConv is a versatile unit conversion tool which supports a lot of units and measurement types.
The program isn't just about basic temperature or distance conversions, then. It also covers energy, mass, power, pressure, and more, an impressive total of 613 units in no less than 21 categories.
Obviously this can sometimes get, well, a little complicated. And if you're never likely to convert Megabecquerels into Millicuries (or whatever) then some sections will seem entirely irrelevant.
But of course you don't have to see a particular area unless you click on it, and in general use ESBUnitConv is extremely simple. It's portable, so there's no installation. All you have to do is choose your "From" and "To" units, enter your "From" figure, and the program will display the result for you (a "Copy" button will then send this to the clipboard with a click).
If this isn't quite enough, though, the author also produces a commercial Pro version with more measurement types, more conversion options, the ability to hide unnecessary units, better customisation, improved clipboard support and a whole lot more. You can read more on the ESBUnitConv Pro web page.