Mathematics
Math major in college, fractals on the side, and a soft spot for Numberphile.
I love math. I was a Mathematics major in college, and I've never really stopped noodling on math concepts since.
Fractals are a particular favorite — there's something about getting infinite complexity out of a small rule that keeps me coming back. I also love a good Pi calculation; the code for my Pi Day calculator is up as a gist if you want to play with it.
When I'm not doing the math myself, I'm often watching Numberphile on YouTube. Brady and the rotating cast of mathematicians are great at finding the surprising stories tucked inside problems most people would walk past.