I'm a big cheap-skate/environmentalist and love finding new uses for stuff most might consider garbage. I just finished off a 52 pound bag of Pedigree dog food bought from Costco. They have begun using a super-strong woven plastic bag tear-proof that just begs to be re-used and not just thrown away. The material it is made of is very similar to the stuff used for plastic tarps, although it is thicker and appears even stronger.
I'll be saving these and using them instead of disposable lawn bags for tossing all of my muddy wet caving gear in on the way home. I'm not sure if other smaller Pedigree bags are being made of this stuff, but I'll check into it as I see them in other stores.