I wrote a script that looks at C++ files, figures out the dependencies (by looking at what headers they include) and automatically compiles and links out of date files. It makes programming in C++ sort of like programming in a modern language like Go. It took me a couple of days to implement and I use it any time I am working on my own C++ projects. I eventually want to add package management features to allow integration with external libraries, but this isn't a priority for me currently.

I have also worked on trading algorithms that used publicly available info about crypto order books to make profitable trades, which will obviously not be published since then they would stop working.