There's Grow Biointensive started and headed by John Jeavons, who actually started the process of answering the question "How small of a piece of land does one person need in order to eat for a year?" back in the 70's. I can't say that I whole heartedly agree with the process, but it's hard to argue with his success. Obviously, depending on land, his current answer appears to be somewhere between 700 and 1000 square feet.
Next, we have Fukuoka, who actually started his Natural Farming technique much earlier. As his starting point Fukuoka used the realization that all of human knowledge amounts to nothing. From Fukuoka we get the question (summarised) "Why is it that plants in nature are vigorous and hardy, but agricultural plants are weak and fragile?". I believe Fukuoka claimed something to the effect of being able to support 5-10 people on a 1-1/4 acre. Not sure about those numbers tho'. Emila Hazelip was influenced by Fukuoka when she developed her Synergistic Garden system.
Last (at least for the big concepts that are floating around), there's Robert Hart's Forest Gardening, which focuses on gardening in 3-D, so to speak. The idea being that you combine canopy trees, understory trees, shrubs, fill in with herbaceous plants, ground covers and, of course, vines trellising along the trees. It's supposed to be a very productive approach, although, I don't think I've ever seen a ratio of human/land proposed for it.
Aside from the ubiquitous Permaculture, those appear to be where the big "after the crash" food production work is, right or wrong.
I personally suspect a mixture of these will ultimately be the most useful, so I'm trying to learn (and practice) anything I can regarding them.
