We also use Clojure at work alongside Ruby, and I'm continually pleasantly surprised to find the benefits of using an immutable-by-default language. I like the functional programming aspects of programming in the large and small, but those advantages pale in comparison to the idea that data should be immutable except when absolutely necessary.

I dabble in Elixir in my spare time, and that language appears to yield similar benefits. Again, immutability is the key, not necessarily the functional focus. While immutability is more difficult to implement in OO languages, I would be curious to see what that looks like.