Your favorite equations.

(Yay! Nerdiest thread evar!)

My personal fav. is Minkowski's "c = i".  I.e. the speed of light is equal to \sqrt{1} as a dimensioned constant.

I thought about this a bit and figured e, i, pi, and all that would automatically be covered, so...

One of my very favorites is a simple result of calculus 1: the integral from zero to pi of the sine of x, is equal to two.  This blew my mind at the time; that a function, with a graph, having a relatively complex definition, resulting in a nice curved shape, should have a natural number's worth of area in each of its humps.  It was a thing that I earnestly worried over in high school before learning calculus, and I was deeply gratified to learn the answer.