I've always wondered what it is, as it makes a lot of noise on being deformed. No other common plastic that I've come across has quite the same properties.
It's more commonly referred to as 'envelope window film'. I've read somewhere that about 90% of all envelope window films are made from polystyrene, but sadly I cannot find the source at the moment. The remainder is made from polyester, polypropylene or glassine.