Oh man, I've got you all beat! ....yes, I just turned coolest formula into a competition, so what?
Except it's not one equation, it's four. Guessed what I mean yet? Maxwell's equations! (in differential form). They tell you everything you need to know about the way electric and magnetic fields work, which extends to imply things like E field always comes out of a conductor at a right angle to the surface, and you can even derive the speed of light out of them.
What a great set of equations!
<img src="http://home.att.net/~newtuniv/max-diff.gif"> ...wow, it was hard to find a good picture of them