Occasionally I find a new idea and wonder how I lived so long without encountering it before. Continuations are one of those ideas. I have been hearing about them for a few months but only recently have I started to understand them. They have been around for a long time and can solve problems that a difficult or impossible to solve otherwise. Of course, continuations are not supported in the in-vogue languages so that is probably how I missed them.
Anyway, here are some links to a couple of tutorials (thanks Charlie). A gentle introduction and a not-so gentle description of external iterators, called generators, in ruby. If those articles do not humble you a little I am impressed.