I was studying neuroscience and met another student who said "Hey, I've been running workshops teaching people how to build simple brain computer interfaces with EEG, but it's hard to explain the signal processing steps without showing them what it actually looks like. We should build an app that does that."
Fortunately, there was a product out there, the Muse EEG headband, with a lot of users and a decent mobile SDK. The company gave us some free headbands to work with, some experience software developers told us what tech to use, and a few months later we released EEG 101: https://play.google.com/store/apps/details?id=com.eeg_projec...)
I'm sure there should be many, many more.