I wired up all my house electrical circuits with a TED[1] unit and wrote code to track electrical usage by circuit.
I've written scripts the automate the creation of parts like screws that can be imported into 3D CAD tools.
I've written code that processes processor data sheets to generate include files for the register definitions.
I wrote a simple CMS to manage web pages.
I've got a bunch of programs that are part of some ML experiments for converting text books into knowledge graphs.
I wrote a simple database to manage all the DVD/Blurays I have so I can know quickly at a yard sale if the movie is one I don't have.
I wrote a 3D graphics package that would talk to the 3DFx libraries for their VooDoo graphics accelerators.
I wrote a filter designer that would optimize RF filters based on standard available parts and a minimum of winding my own coils.
Libraries for STM32 chips so that I could easily go from idea to something I could test.
The list does go on, but you have to understand that before I ever wrote code "for money" I wrote it for fun, learning, and exploration.