Styrofoam - It's not biodegradable.
There has also been concern about the trace presence of Styrofoam's production chemicals remaining in the final product which can be toxic. (i.e., benzene is carcinogenic). There's more info on that on the EPA website.
Additionally, some scientists believe that producing styrofoam can emits ozone into the air at unsafe levels.
The most horrifying thing about Styrofoam? It does not biodegrade. The California Coastal Commission says that it's a huge component in marine debris. It is also bouyant, so they can just float around waterways indefinitely. A hideous, unneccessary, and perpetual eyesore that is terrible for our environment.
Most people don't know this, but Styrofoam actually IS recyclable. Curbside recycling usually won't take it, so you'll have to find local places to drop it off. Check out the search database at Earth911.com to find a recycling center near you that will take Styrofoam. Search by its generic name "polystyrene."