I work with Ruby on the web, but rarely with Rails. Apart from the Ruby language itself -- which I love, even though I sympathize with those who don't -- I can't imagine giving up these two libraries by Jeremy Evans:

[Sequel](https://github.com/jeremyevans/sequel) - Sequel is the best ORM I've ever used, in any language. Fast, stable, unbelievably well maintained, and offers great low-level access to the database when you need it.

[Roda](http://roda.jeremyevans.net) - This dynamic routing library is conceptually similar to React Router, in that there is no static list of routes; routing is a function call. But it's faster than React on the server, and has better support for HTTP verb support, database I/O, etc.

As for there being "better tooling" in modern JS, I'm not sure I agree. Babel and ES6 are impressive, and I love writing the code they make possible. But I appreciate being able to write Ruby and know that it will just run. Writing packages for node means choosing one of two incompatible module systems, or adding transpiler bloat.