I'd second Erlang. It's a lot of fun first learning. I know when I went through Armstrong's book on it, the first 5-6 chapters were mostly just "okay, so an intro to the syntax and functional programming", then when the meat of the language hit, it was a big fat "whoa...this is crazy."
I'm glad HN had that crazy "Erlang Day" a few years ago, as that was Erlang's first blip on my radar. Thanks HN :)
Erlang is definitely the most fun language I've learned in many years. It will make you think very differently then most other languages and at least slightly change the way you think about everyday programming.
