This comment of Dave Bell's at Charlie Stross's left me thinking of something. Bell refers to a pocket handbook of crop yields and other agricultural data. I've always liked this kind of thing - having the data on hand for anything, however weird. Somewhere around I have a copy of Every Boy's Handbook as given to my dad when he was a little 50s boy, full of useful data on Newfoundland and the De Havilland Comet K Mk.1.
In the light of this megathread, I wonder what you would include in a data manual for the contemporary world (a real User's Guide to the Millenium..), perhaps the help file for the "shrink-wrapped military-industrial complex" Stross mentions, or more realistically part of the package with the RepRap, DNA synthesiser, printed solar panel kit and Inmarsat BGAN terminal Thomas Barnett might want to drop on Karachi, in between bombs.
One answer would be "it's called Google", of course. But there's a lot of cruft out there. Say, instead, we're implementing it on a smartphone. We can pack a scientific calc package too, perhaps one of the Python maths libs on a Nokia S60 device. In a 4GB SD card. What would we include?