Polylactic Acid, also known as PLA, is a common biodegradable thermoplastic polymer.

PLA or Polylactic Acid is a biodegradable and bioactive thermoplastic aliphatic polyester that is the most common plastic to begin 3D printing with, since it can be printed without a heated bed. It comes in various colors, typically on spools between 500 g and 3000 g.

"PLA is harder than ABS, melts at a lower temperature (around 180°C to 220°C), and has a glass transition temperature between 60-65 °C, so is potentially a very useful material. It does exhibit higher friction than ABS however which can make it difficult to extrude and more susceptible to extruder jams." (RepRap Wiki)

It is "derived from renewable resources, such as corn starch (in the United States), tapioca roots, chips or starch (mostly in Asia), or sugarcane (in the rest of the world)." (Wikipedia entry)