Unit Converter Expert enables you to quickly perform conversions between literally hundreds of measurement units.
There are many things I like about this little tool: first, you don't need to install it; second, the measurement units are conveniently classified by areas and it makes the program really easy to use. Finally, conversion is done on the fly, without having to press any key or click on any button. In fact, there are only two buttons: one to launch Windows calculator and another one to copy conversions directly to the Clipboard.
The only thing I found to be a bit annoying is the fact that Unit Converter Expert doesn't show on the taskbar, so if you lose track of it you'll have to use the Alt-Tab shortcut. Another option is using the program's "Always on top" option.