[fmw]

Beware of people suggesting their personal favorite language. There are many languages worth knowing. My personal favorite is the Lisp family and particularly Clojure. Pick up a book like SICP or any of the Clojure books. It can really give you a new perspective.

I suggest buying the Seven Languages book by Bruce Tate. There is no wrong choice, because learning any language will make you better.

[RutZap]

I'll definitely take a look into a Lisp language. I guess it will also make me learn some maths (lambda calculus) (one can never know too much math :D)