Control nanobots crawling about your heart trying to eliminate plaque, cooties, and tears. I worked with a team of six, and unfortunately I was heavily outvoted when I wanted to put toxoplasmosis as one of the things infecting your heart ("the word doesn't fit on the screen", they said). We were all terribly impressed with ourselves with this one, and put it on the app store. Unfortunately, I haven't renewed my iOS subscription, and it now currently off the store. Written in Objective-C/Cocoa, iPad only.
Update! It's back on the iOS store! I even updated it to work on modern iPads!