There is this Polish guy who has been working on repetition spacing for a while. He seems less into usability, though, and I think there is an opportunity here for someone who wants to make this kind of stuff simpler. <p><a href="http://supermemo.com" rel="nofollow">http://supermemo.com</a><p>(There are some interesting articles on sleep too, if you are not intimidated by walls of text.)<p>In my experience, repetition spacing seems to work for memorizing stuff. I'm less sure about learning skills- operative knowledge-, as opposed to passive information.