Two things:
First of all, holy search engine optimization! Of 1,000,000 pages for 'pretty equation', this comes up third in google!
Second, check wikipedia for natural laws. Here's two good sources:
Laws of science - Wikipedia, the free encyclopedia (some basic fundamental equations that most people with undergraduate science education would recognize)
For some 'prettier' equations, check:
Noether's theorem - Wikipedia, the free encyclopedia
Noether's theorem equations has some artistic elegance to me, because they are different ways of stating some natural laws, to allow for more variability. So, they're not mathematical operations as much as descriptions of the natural world.
