I’ve just come across this (thanks to MaxOSXHints):
Type a mathematical expression into the search box in Safari (or any similar Google-enabled program) and you get the result. Works for simple stuff like ‘1.4 *2.2’ but also understands some quite fun stuff:
36 * 17.5% 2 * pi * radius of earth 0x44 in decimal sqr(-16)
and even:
e ^ (pi * i)
Update: more fun from Kuro5hin