Expression Calculator. Uses the same expression parser as DataTank.
You type in an expression in one field, press enter and get the result in a second field. The expression you type can be fairly general, and contain all the standard functions and operators. Expressulator defines a large number of constants and allows you to include an unknown in your expression, defined by another expression.
You can open any number of these windows with Command-N, or with Command-Shift-N create a compact version
The windows will float above all normal windows.
Expressulator is a very small program, so for further explanation just download it and try it.