This is from the realm of mathematical physics, but Noether's Theorem (http://en.wikipedia.org/wiki/Noether%27s_theorem http://en.wikipedia.org/wiki/Noether%27s

This is from the realm of mathematical physics, but Noether's Theorem (http://en.wikipedia.org/wiki/Noether%27s_theorem http://en.wikipedia.org/wiki/Noether%27s_theorem) is quite possibly the most beautiful result that I can think of: Briefly, "Any differentiable symmetry of the action of a physical system has a corresponding conservation law". The extension of this theorem in QED, The Ward-Takahashi Identity, is also incredibly astounding, if a bit obtuse if you're not well versed in renormalizable field theories.