I created a niche CLI tool to clean messy CSVs. It was too small for a startup, so I made a simple landing page. Then, I shared it in forums and added a 'buy me a coffee' link. To my surprise, it brought in small but steady income. You can also bundle tools into a digital product (like a 'developer toolkit') and sell on Gumroad. APIs and microservices on RapidAPI or GitHub Sponsors also work if your tool solves a real pain point.