Or a set of machines (appliances really) to take one back in time (at least mentally).
This post from William Turkel at Digital History Hacks about the history appliance was just down-right awesome. How cool would it be, even if just in your browser or on your computer, to have everything (all types of media) display a certain time period or year.
What I thought was most interesting is the immersion effect this would have for historians. It would be a great way to see what was available at that time, what was not, what was popular, what was in the news… Pretty cool.
