I think you could get basic metallurgy, rocks, minerals, factory techniques, description of various machines, motors, engines, pumps and irrigation, stills, solar stills, horticulture, animal husbandry and other things from wikipedia at a level that would be useful. Things like "is this plant edible" are potentially useful.
I see where you're coming from. I just feel like so much of wikipedia is pop culture and history (I could be wrong though, I have no idea how to check this) that there would probably be a much more efficient resource for a post-apocalyptic scenario.
