My favorite so far was a company I founded about 20 years ago with a friend who worked in the film sound industry. He had an idea for a hyper-niche tool for automatically syncing changes from video to sound (industry term is "conforming"). He'd approached a couple people/companies he knew that made tools in adjacent spaces but got turned down with "that's impossible" or "it's not worth it". I had no experience in the space but I got intrigued just from a "can it be done" perspective and coded it up. Once written it required very little maintenance, it was just available on our website for download and we used a shareware service to deal with licensing and sales. Only marketing was word of mouth. Every time someone bought a copy the shareware service would send a text, so I set the text alert sound for their number to be the cash register sound. It was a small addressable market so didn't sell all that often but it was always a little dopamine boost to hear the "ka-ching" come out of my phone. Most sales only netted me a couple hundred bucks, but every once in a while a big player (like a movie studio) would pick up a bunch of copies at once. Eventually sold it off to a competitor and collected a small royalty stream for another few years.