PLANTS FOR A FUTURE
  http://www.metalab.unc.edu/pfaf/D_search.html
A Resource and Information Center for
Edible and other useful plants

This is what you can do:
+Search by Name, Common Name or Family.
+Search for an Edible, Medicinal or Other Use.
+Search for plants native to a particular Area.
+Search for plants which grow in a particular Habitat.
+Search every field for a particular Word.