While I was reading i : the Imaginary Number, I learned this (a.k.a. Euler's Formula):

e^iπ = -1

Like the author of the linked article, I think it is pretty interesting that e, i and pi are linked that closely. There's a proof here, which includes this bit:

Gauss is reported to have commented that if this formula was not immediately obvious, the reader would never be a first-class mathematician.

Obvious? I don't know if I could puzzle my way through it in a day. I wish I was better with math.