Midget

A tiny, but very capable numerical calculator for Windows. It differs from others in the way you calculate your expressions: enter the whole expression at a time, like "2+3.9/7.1-3 sqr 5", and only then hit Enter to compute the value.

The expression stays displayed so that you can correct or othwerise modify it, and recompute at will without having to re-enter everything from scratch.
Calculations are done with 18-digit accuracy, and the results can be displayed with adjustable precision and format (including deg/min/sec). Results can be also converted between various units of measurement.
In addition to basic arithmetic operators and 30 standard functions, Midget has 12 variables, where calculation results can be stored, and then used as arguments in other expressions, like in "2A+B/7". The status of calculations is saved to disk so that next time you run the program, you start at the point you left.
The program is continuously improved since 1996, so it reached a highly polished state. A Help file is included, and there is a Web support page.
If you are still using the crummy calculator applet included with Windows, it is time to try Midget. The chances are you'll end up using it on a permanent basis.