Really depends on the side project. I'd pick C if writing low-level/kernel code, OCaml for language-processing tools, Python for scripting/ML/Algorithms. Go for

Really depends on the side project. I'd pick C if writing low-level/kernel code, OCaml for language-processing tools, Python for scripting/ML/Algorithms. Go for distributed systems. It'll also depend on available libraries. Occasionally C++ if I need speed + data-structures. But most of the time, my side projects are just a pretext to learn something new, so I would happily pick up a new language. As for the tools, I use VSCode that works well with all languages, and git (github/gitlab, probably gitlab). Then VMs or docker. And unix shell/bash.