We all know this one. It has an unmistakable form and you have seen it in magazine ads, movies, and other media for your whole life. Well the best part is its a relatively affordable plastic chair. If you've never sat in one it can be a little deceiving. They sit very well, with just a little flex that makes them comfortable for more than a short span.
Originally these were made from hard fiberglass shells, but many manufacturers like Vitra have moved on to a polypropylene shell. Sometimes you can get a bargain on a used set, but if you look at an old one you'll be into collector's territory and the prices will shoot back up.
These are widely available and it seems there must be some copying going on out there. Here is info from Vitra's site about the chair:
Because of technical progress in plastics processing, Panton Chair has been through a number of production phases since its original launch. The last version of the chair authorized by Verner Panton was produced in collaboration with the designer at the end of the 1990s. This model realized one of Panton’s fundamental objectives for the first time: a plastic chair as an affordable industrial product.