Someone posted aloe, and that they weren't sure what its for: minor burns.
The wonders of tea tree oil have been discussed, but don't forget that mixed with a little water and citrus (lemons, or oranges for example) it can
be used as a multi-surface cleaner.
Two other pdf books that are available for free that might be useful in a "total breakdown of society" type of situation are: "Where there is no
doctor" and "where there is no dentist". There are also other variations, one targeted specifically for women.
Pine needle tea= excellent source of vitamin C.
Willow Bark tea= aspirin equivalent, good for headaches!
Kudzu= invasive plant that's widely available and edible
Cattails= the heads can be ground into and used as a flour sort of substitute, for making breads and whatnot
Dandelions= The greens are edible, and they're everywhere.
Broad-leaf plantain= Another widely available, and edible weed.