I'm always curious as to how side projects that are so niche and targeted acquire users. I would like to ask the author how much time and effort he has dedicated to marketing and what he learnt from it.
This may be the hardest part, especially for technical founders. I was able to bootstrap a profitable subscription based service. In hindsight most credit should go to my wife for getting in the first users by engaging family, social media, forum posts and what not. Another major factor were my subscribers, who write site content on a daily basis, this generated a ton of organic traffic to my site and made a big difference vs having to write a couple of static SEO optimized pages and hope for the best. Writing the actual code was probably the least important part (but the most fun for sure)
The easiest way is to build something for an existing community where you are not disallowed from promoting your own stuff. Ideally you need a way to talk to a lot of target users without having to pay money on advertising.
As for ones that manage to gain users without latching onto an existing community, I'm wondering as well. Perhaps good SEO or searchable content generation?