Node.js -- Can easily build a ton of server side and CLI tools with ease. Saved me a ton of time!
When I first tried NodeJS, I was hooked... You can build simple CRUD backend APIs within minutes! I'm aware you can do the same with Java, e.g. something like Spring Boot streamlines the process a ton as well, but the feedback loop of just writing a line or two and having a server automatically reload within seconds is amazingly addicting.
I am not sure I would use Node for production... I suspect a lot of the Java world might switch to Go as being much faster yet much safer than something like Node.
(I am an amateur Python dev and tried Go but could not live with the error handling boilerplate and the lack of exceptions. It looks like a nice language though)