I'd really like to learn assembly language, for no better reason than to satisfy my curiosity.
I bought a book on it a while back and I found it quite amusing that they only get to the "Hello, world!" example by about page 400.
Also, it's not really a programming language but I'd love to check out Arduino at some point.
You can use Ruby, Lego and Arduino to build a robot (of sorts). How cool is that?