The title says it all: for every four packs of Polaroid 600 film you buy, you get one free. To get free shipping, use promo code ERV2YB during checkout.
The offer ends 28 September, 2006, and is only valid if you buy directly from Polaroid.
I bought a crapload when they had the same offer on Spectra film a few months back and my order was correct and arrived quickly.
(This is not an affiliate thing, I got this in Polaroid’s newsletter.)