I picked up Erlang a couple of years ago because I needed to hack on a Jabber server. Never mind the functional aspect; Erlang stands out for concurrency, distribution, and fault tolerance. Several of my Erlang projects have been running, at scale, trouble-free for months. I get a kick out of never getting service alerts.

Lisp is another good answer to your question. It's different. And the cliché is true: even if you never use it, your coding will improve if you understand Lisp. (Land of Lisp is a fine primer.)

However, with Erlang it has been much easier to find actual projects to do.