Spotlight is that little magnifying glass in the top right corner of you screen. It is a great way to quickly search for file and applications on your Mac, but I like to use it as a calculator.
It's really simple and fast. Just click on the magnifying glass (or use the keyboard shortcut command + Spacebar) and type in a math problem. For addition you use the +, for subtraction us -, for multiplication use *, and / for division. You can even use parenthesis' for a math problem like 78+15+(54/24)*24 (the answer is 147). Sure, it won't do your advanced calculus for you (see WolframAlpha for that) but it is a great quick fix when you need it.