Learning software development through playing and contributing to a MUD:
miniblog.
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tree-sitter has been amazingly successful at getting developer tools to work together.
Neovim has a bunch of people contributing to parsers when they notice highlighting issues. That benefits the whole ecosystem: semgrep, difftastic, even Emacs!
Neat GitHub feature I haven't seen before: if the contributing guide has changed since your last pull request, it tells you!
Valve has forked and packaged up Wine to help Linux users play games: https://github.com/ValveSoftware/Proton
This is FOSS working as intended: freedom to try other things, but contributing upstream!

