I've been working on SoundFingerprinting [1] for almost ten years now. Its Shazam for developers. First 8 years I worked mostly during my free time and weekends. 2 years ago, I decided to monetize the storage for the fingerprints. There is a MIT licensed version of the storage [2], and a commercial one [3], which is fine-tuned for thousands of hours of audio or video content. If you are an enterprise customer, you will essentially need it at some point, and will most probably have no problems paying for it.

Overall I invested a lot more time in it than the monetary reward I received. I don't complain since I enjoy working on audio/video fingerprinting and databases. On top of it, the pay at this stage is on par with a regular SE job with the bonus of working on things that I enjoy. I can call it a bootstrapped business now.

In some sense, I think about opensource similar to the work of art. You do it because you genuinely like to create/build things and showcase them to the general public. You don't do it because of the monetary reward. A good writer is one that has to say something, not one who writes for the sole purpose of getting on the NT bestselling list. Opensource is similar.

[1] - https://github.com/AddictedCS/soundfingerprinting

[2] - RAM-based storage, bounded by memory limits

[3] - https://emysound.com