I made
https://distilledjs.com/, it's a Javascript testing library with no dependencies, the entire API is only 3 methods (only 1 of which is really necessary to learn), and it very deliberately doesn't ship with a lot of standard features.
Distilled is based on an evolution in my personal philosophy about how effective testing works and about how good APIs are designed in general. My goal building it was to have something that is extremely flexible, but that is also so simple that I can start using it within about a minute of setting up a new project. When I start writing Javascript, I don't add a testing framework immediately. Instead, I import Distilled and write my tests in the same file that I'm writing my code in. Later on, as the project evolves I start to separate things out and build a more complicated testing setup.
I have obviously put some work into documenting it and making it available to other people, but I don't need anyone else to use it. The project is already mature enough that I am using it in every single personal project I maintain. It doesn't need a big ecosystem, it doesn't need a big community. I haven't touched the API in probably close to 2 years.
I will likely use Distilled for the majority of all of my personal projects in Node for all of the foreseeable future in my development career. I've used a lot of testing frameworks, and for my particular needs I think that Distilled is the best testing library available for me to use. It has saved me an enormous amount of time, and it makes it easier for me to do complicated things with my testing suites, which has made developing other personal projects much easier. If nobody other than me ever picks it up or uses it, it doesn't matter. It's already paid for its own development costs in personal time savings on other projects by an order of magnitude, and I think it will continue to do so in the future.
I think it's one of the most personally useful pieces of software I've ever written in terms of pure benefit to my own development processes, and (while I'd be happy to see more people use it) I don't think that having a community around Distilled would change anything or introduce any giant benefits. I'm not going to add a bunch of extra features or methods to the API. It's really stable and extensively tested; I've been using it for years and I'm not aware of any bugs that need to be hunted down that a community could help with. So if other people want to use Distilled, great, but them using it or ignoring it changes nothing about how useful it is to me.