Useful Conversions Page

This unit conversions page is useful because it also supports areal and volumetric conversions.

On a related note, unit conversions are possible inside of Arcmap as follows. "When editing in ArcMap, you can do on-the-fly unit conversion if you specify the unit type when you type a distance (by right-clicking). When you enter a distance into a pop-up window, press F1 to get a list of the unit abbreviations you can use." (Arcmap help)