Some things I built on the side, outside of my day job:
In the late 90's, I sold a command-line SMTP e-mailer for Windows. It was easy enough for folks to integrate e-mail transmission into their systems ... even 16-bit systems since spawning a copy of the shell would allow 16-bit systems to invoke my 32-bit mailer. Lots of folks had used these tools for all sorts of things. I got registration checks and cash from around the world before I started taking credit card payments.
I have an open source command-line MP3 player for Windows that folks still use and incorporate into their systems, JS libraries for node.js, ...etc.