In a review of Green Plastics: An Introduction to the New Science of Biodegradable Plastics, Dave Aftandilian gives a pretty decent introduction to bioplastics:

"It turns out that there are a surprisingly large number of sources of natural polymers just waiting to be exploited. Starches, for instance, are abundant and cheap. Cellulose would also be a good source, as it is so widely available in plant parts -- in fact, using cellulose could turn common agricultural wastes such as straw and corn stover (the stalks, leaves, and husks left over after the cobs have been harvested) into valuable feedstocks for plastics, giving farmers some badly needed additional income. Currently it is difficult to extract cellulose from such wastes, but research is ongoing. (Perhaps some of the bacteria that digest cellulose in the stomachs of cows, goats, and other ruminants will provide a clue.) Various polyesters can also be obtained from bacterial fermentation."