Salt is one of the world's oldest forms of payment. In fact, the word salary derives from the Latin "salarium," which was the money paid to Roman soldiers to buy salt. It was the main form of currency in the Sahara Desert during the Middle Ages, and was used extensively throughout East Africa. Typically, one would lick a salt block to make sure it was real and break off pieces to make change. Doty's block, seen here, is 1,500 years old.
Other incredible, edible currencies include "reng," a yarn-ball of tumeric spice wrapped in coconut fibers that is used for trade in the Solomon Islands; cacao (or chocolate beans), widely used throughout Mexico and Central America; and Parmigiano Reggiano cheese, so highly regarded that it was used both as currency and bank collateral in Italy.
One particularly inedible currency: the poisonous money seeds of Burma. Which if nothing else proves that money does indeed grow on trees — or at least bushes.