[cgag]

Clojure or Haskell would be my choices, depending on how into static typing you are or how interested in lisp you are. I really would want to learn both, and if that's an eventual goal, I think Clojure might be an easier first step.

[sejje]

I second this. I'm pretty proficient in all of the typical languages in web development...PHP, Python, Ruby.

I started SICP, but followed along in Clojure, and I've been having a blast, plus my eyes are opening to a lot of things that I needed someone smart to tell me.

I don't have plans to ship Clojure in production, but it has still shaped what I'm doing on a daily basis in other languages.