crocheted plarn bag by Thoughtful Rose, she used loops of plarn, and a size H crochet hook. Tight, dense stitches are the key to a plarn bag that won't stretch out of shape (that, and crocheting through both loops). Plarn is yarn made from plastic bags
How to recycle plastic grocery bags into reusable grocery bags, if you know how to crochet. Great instructions for how to make "plastic bag yarn." Plastic bag yarn: one of the few uses of plastic that actually make sense.