All you need to do is press Cmd + Space, and then just type in the math. Spotlight can do simple math that calculator can do, but don't expect it to do anything involving variables and graphing. But you can do stuff like:
and even more interesting stuff like:
Pretty neat, huh? At least you don't have to use the calculator application or the calculator widget. And yes, putting the Canadian flag over there in the last screenshot was totally accidental =P
Got a question, tip or comment? Send them to firstname.lastname@example.org and we'll try to answer it in a blog post!