EKG

I love reading EKG's. I think it's incredibly fun. The basics are easy to grasp; a beginner can at least make some comments on a given EKG. But there's also a beautiful complexity to it, that with finessed experience and more care, one can decipher so much about the anatomy, physiology, and pathophysiology of a person's heart. For a simple non-invasive test, it harbors remarkable potential.

Image is in the public domain, from Wikipedia.