What's Your Favorite Math Equation And Why?

Back in 2004, Physics World polled their readers asking what was the greatest math equation ever?  As Joseph Polchinski has recently noted:

[They] came up with a two-way tie between Maxwell’s equations

and Euler’s equation

The remarkable appeal of Euler’s equation is that it contains in such a compact form the five most important numbers, 0,1,i,π,e, and the three basic operations, +,×, ˆ. But my own choice would have been Maldacena’s equation

                                         AdS = CFT

because this contains all the central concepts of fundamental physics: Maxwell’s equations, to start with, and their non-Abelian extension, plus the Dirac and Klein-Gordon equations, quantum mechanics, quantum field theory and general relativity.

All good equations and I must say I stand in awe of how much symmetry, beauty, and information they pack into one line.

That said, my favorite equation is Stokes Theorem written in the full differential forms machinery:

Look how symmetric!  This equation both generalizes the fundamental theorem of calculus and illuminates the heart of the duality between homology and cohomology groups in algebraic topology.

Now, please share your own equations and why you like them!  I'm interested in what your favorite equations are, whether you enjoy them for technical reasons or not.  Even if your equation is something like:

One egg + one cup of sugar + one cup of peanut butter + 10 minutes at 350 degrees = a surprisingly good set of peanut butter cookies.

Please share.