Some time before I began studying computer science, I heard of the book "structure and interpretation of computer programs". As I was always on the search for new, life changing computer languages (as someone once said: "A programming language which does not change the way you think about programming is not worth learning"), I was quite eager to get my hands on the book. However, it was out of print back then, or at least impossible to get in Germany for an agreeable price.
So I started to learn scheme from the official standard document which turned out to be rather painful. Anyway, years passed by, I learned C++, finally got back to C, learned python, then ruby, played a bit around with haskell, and emacs lisp. And suddenly I learn that the whole book is online.
Well, turns out that ten years later things are maybe not that exciting any more, but still, maybe this book can still change your life ;) Who knows what I'd be doing now if I read this books back then... .