I'm the solo founder of Holiday API - https://holidayapi.com - a DaaS platform for holiday information.
The project was originally started in 2013 as an open source side project, while I running a network of niche social networks full-time (also quite profitable for a while), after serving as CTO of a daily deal company, that stemmed from my need for reliable holiday information without the overhead of actually maintaining said data on a regular basis.
A shift to a premium model was made in 2016 and the first month into being a premium service, it was profitable (sans my time ;) and has grown to ~$1500 MRR.
Ironically I started the project as a way to NOT have to maintain said data and now part of my day to day involves maintaining data accuracy... and sales outreach... and DevOps to improve... my own development efforts... and... and...
Recently received my YC rejection email for the upcoming batch (actually interviewed with another project with a partner a while back), but still hustlin'. Flying solo is great, but definitely can't get caught up in the echo chamber. Would highly recommend building a peer group of founders / other hackers & hustlers to meet / chat with regularly.