I often find myself in the situation where I need to do some quick math. Calculator.app and the calculator widget are pretty basic and a pain to use as they try to mimic the paradigm of a physical device.

There are other ways…

Spotlight

Did you know you could use Spotlight as a calculator? Just type away and it’ll give you the result instantly.

terminal.app (bc)

I’m a regular terminal user. Most of the time I have at least two terminal windows open and guess what there’s a command line app for calculations. It’s called bc.
Just type bc into to prompt and hit enter. Now you can type things like 2+2, or 2*(20-40)^2. Enter.
If you’re done, type quit and hit enter. bc can do a lot more than that, you might want to take a look at its manpage.

AppleScript

Another option is AppleScript. There’s a pretty handy OS X service, you can use. Open System Preferences -> Keyboard -> Keyboard Shortcuts and mark “Get Result of AppleScript” under Development.
Now you can use any text input area as your calculator, just type and select the term and press ⌘*

General things

You’re allowed to use brackets.
Use sqrt(n), or (n)^(1/2) to get the square root of n
Use x^(1/n) to get the nth root of x