A few years ago, Greg Wilson mentioned that, a useful feature when teaching students would be the ability of recording programs and have the possibility of playing them back one step at a time either in the forward or backward direction. Actually, I am paraphrasing what Greg said or wrote, as I don't exactly remember the context: it could have been in a web post, a tweet, or the one time we met and when I gave him a brief demo of what Crunchy was capable of doing at the time. While I can not say for sure what Greg said/wrote and when he did it, the idea stuck in my head as something that I should implement at some point in the future.
This idea is something that the Online Python Tutor, by Philip Guo, makes possible.
It is now possible to do this with Reeborg's World as well. :-)