GitHub Codespaces
GitHub Codespaces is like GitHub.dev on steroids. While GitHub.dev lets you edit your repo in your browser, Codespaces is your own personal cloud-based development environment. I'm still only playing around with it, but I can see how it has potential to be a very powerful tool.
At a super-high level view I would say it's a bit like containers, but for a whole development environment. It lets you configure the environment for all your users, while still allowing indiviual customisations. A new team member can have their own Codespace on day one, with access to the default team settings, configs, build tools, etc...
I need to think some more about how I will use this for my own projects, but on a team level it's a definite win!