One thing I've often wondered: with the knowledge we have now and the technology available to the nineteenth century (or maybe up until about the 1930's say) could we go back and reinvent mechanical or valve driven computers, only make them much faster than they knew how then? (Eg like an updated version of the Analytical Engine). Would come in handy if a world-wide catastrophe occurred and we were all plunged back into the dark ages in terms of industrial capacity but we still had the knowledge. (Provided we remembered to make a hard copy before the asteroid struck :)