For me, it would be Caddy Server

Discovered it while running some researches around GoLang, where I could see that it was really useful when used with Hugo Framework.

It took me something like 10mn to configure it to run (I couldn't believe only 2 lines would make my things work), and 5 more to make it run with Docker and Traefik (another good open source project that I now use everyday).

Now all my projects where I use Docker have Caddy included o/