Ask HN: Who has a niche side project?

HN is keen on side projects and threads always have plenty of us dropping links to their own and being very open.

There is often the suggestion that people with small niche projects will lurk and not contribute to those threads for fear that someone might compete with them - that they have some secret market which the rest of us don't even know about.

I'm interested in hearing from HN users in this situation who without talking explicitly about their business/domain/niche can talk about how they came to capture some small niche in its entirety and monetise it.

Anyone?