Visual Studio Code as a Progressive Web App? (e.g. works offline, with at least partial functionality).
An impressive achievement, and has some interesting usage scenarios. One to remember when Vue support is added (coming soon) ;-)
Still, the extensions available for VS Code are just excellent in terms of productivity - so it's not currently a 'replacement' from my perspective.
I could see this being very, very useful for training / presentations, where each attendee can fork it and walk through an exercise, without any worry about having the same set up. (Hint to Sarah Drasner )