There are all kinds of cool geeky apps that engineers can have fun with.
On my phone, I have:
* Seek Thermal app, for thermal camera
* Barcode scanner, to quickly nab bar codes off products etc.
* Spectrum analyzer app, uses the phone's microphone to display a spectrum of the sound around you
* OBD scan app, connects to the car so you can see status and trouble codes from the engine, transmission, hybrid battery etc.
* Barometer... great for tracking cabin pressure on board a plane too
* Compass... great for finding N and S poles of magnets as well, if you work with Hall sensors and the like
...And so on.
If you can think of it, someone's probably already written an app that does it!