Recently I've been reading this (http://math.ucr.edu/home/baez/einstein/einstein.html) link talking about general relativity, Einstein's theory of gravity. Relativity is infamous for using some very gnarly math that made even Einstein's head hurt, but this link is a great reminder that relativity is a fundamentally physical theory that you can understand intuitively. It just happens that, once you understand the intuition and need to do a calculation, tensor calculus on Lorentzian manifolds is the "natural" way to do it. But as this link shows, you can state relativity without using heavy-handed math, and you can derive at least some results without the full mathematical formalism. Cool stuff.
This blog features a series of mini-essays about the history and nature of math, and how it fits into the spectrum of human activity. I'm considering putting them together into a book, so comments are encouraged!