SpeedCrunch is a keyboard-based scientific calculator which is easy to use, yet still has the power that advanced users need.
The program is both open-source and portable, which means no real setup hassles. Just unzip the download and run speedcrunch.exe to begin.
The default interface keeps distractions to a minimum, with just a line to type expressions and a history window to view the results.
There’s an optional numeric keypad (View > Keypad) if you prefer clicking buttons to typing, although it'll probably just slow you down.
SpeedCrunch isn’t strict about its own syntax. It doesn’t mind if you type "2+2" or "2+2=", you'll get the same answer as soon as you hit Enter.