I built https://feedmail.org because I wasn't satisfied with the existing RSS-to-Email services available.

I figured I would really just be making it for myself but decided to make it a product and have made a few hundred dollars in the first months. It pays for the hosting costs and gives me some motivation to work on it. It's also been surprisingly stable and my monitoring is good, so if I am not actively working on it I can just walk away and forget.