http://www.svetools.com/numculatorNumculator is an always at hand, fast and customizable calculator, which can be activated just by NumLock key(to use numeric keypad) or by other hot key. It appears on screen when you want to calculate something fast, don't need to search for and ran any program. Supports history of operations, skins, font and size customizations, different formatting of numbers, pasting result to documents and more.
Added here: http://www.portablefreeware.com/index.php?id=2617