Learning another language

I've been meaning to learn another programming language as a way of 'sharpening my tools', so to speak. For years I've had my eye on LISP, but wasn't sure it would be worth my time, given the predominance of the MS .net stack, Java, javascript, and some of the new scripting languages - ruby, python, etc. But I stumbled across a recommendation to watch a series of videos on LISP from MIT professors Hal Abelson and Gerald Jay Sussman.

They are freely posted here: http://www.archive.org/details/mitocwsicp

Well I tried the first lecture, and while I chuckled at the dress and hairstyles of the early 80's, I was quickly hooked on the content. I quickly realized that this wasn't just a course on LISP, but rather a great course on the really great concepts of computer programming. I'm on the 3rd lecture so far, and having a blast. This might be old-hat for some CS grads, but I'd still really recommend checking these out if you want to geek out on some advanced ideas.