Numculator is compact desktop calculator with some interesting and unusual features.
The program runs in the background, for instance, by default popping up when you press the Num Lock key. The interface is just a one-line text box, no buttons to click, so you can start calculating immediately by typing on your numeric keypad.
Numculator supports a basic range of functions and math types: Abs, Sqrt, Exp, Factorials, Logarithms, trigonometry, Min/ Max, Binary/ Decimal/ Hex conversions, simple date math (date1 - date2 = time elapsed, date1 + time elapsed = date2, more). Check the documentation for the full list.
Everything you enter is recorded in Numculator's history. Reviewing this is as easy as tapping the down arrow, or you can save the entire history as a text file.
Once you've finished your calculations, pressing Ctrl+Enter immediately closes Numculator and pastes the final figure into the previous document. (Of course you can also copy any intermediate figure to the clipboard, as usual.)
Perhaps best of all, just about every aspect of the program is configurable. You can tweak its formatting rules, rounding, alignment, font, transparency, hotkey - there are even 20+ skins to choose from.
Numculator has only limited expression support, but it's configurable, easy to use, and if your math needs are simple then it'll serve you very well.