Euler's identity blew me away when I first learned it a year ago:
e^iπ + 1 = 0
e^iπ + 1 = 0
* the ratio between the a circle's circumference and diameter
* the number such that the derivative of e^x = e^x
* the square root of negative 1
Mind. Blown. The universe is one crazy place.
o, and another cool one is (1/2)! = sqrt(pi)