I found out this isn't as well-known as I thought, and it's pretty useful, so I'm sharing it here.
You can use Spotlight in OS X as a calculator by typing in the calculations. If you're already typing something, you don't even have to leave the keyboard - ⌘+spacebar opens the Spotlight query, and you can start typing straight away.
Hmm maybe this isn't as well-known either. You can also use Spotlight as a dictionary, by using the same method.