I run a number of tiny services in this category, such as an autoresponder for Google Hangouts: https://gchat.simon.codes/. I originally built it for myself - I forward many gmail addresses to one and was missing chat messages - but these days most of my customers are small business owners or employees of larger companies.
I recently summarized all my projects and my numbers here, if it's of interest: https://www.simonmweber.com/2019/01/07/side-project-income-2...