1) oScope — an oscilloscope in your pocket. https://itunes.apple.com/us/app/oscope/id344345859?mt=8
2) Octave — a real-time audio analyzer. https://itunes.apple.com/us/app/octave-an-rta-for-the-iphone...
3) Fourier — a spectrum analyzer. https://itunes.apple.com/us/app/fourier/id386084557?mt=8
I built all of these for fun in college, and I've occasionally updated them afterwards. The only thing I do now is answer a few emails a week. I've since gone back to grad school, but the yearly income has not changed, and approaches my stipend (low 5-digit).
What's been really neat is how people have found unexpected ways to use the apps. Sound engineers for halls and communities use Octave to set up the sound for concerts. Teachers use oScope to help kids understand how sound is composed of moving pressure waves of air, and how pitch is the frequency of these waves. Also, oScope had a tiny cameo in the show Homeland, as a "fancy science-looking analyzer tool for spying on people" (uncredited, unfortunately).