Onya bags are the most functional reusable bag yet.
Made of strong STRONG parachute silk, with a useful pleat to create a wide-based bottom (this is note a tote-style reusable bag with a narrow base), Onya bags are designed to be comfortable carryed by hand or, if you prefer, on your shoulder.
There's a useful pocket in the front too.
One other neat trick is the caribiner / keychain that comes with every bag. To never be without a bag, use this as your keychain, hook it to handbag or to a belt buckle. Because the bag is so delishly squishy, it can even be stuffed into a back pocket. You can also use the caribiner to attach a few bags together in preparation for your next shopping trip.
The best part of this bag though is that there's no folding required ! (well, you can if you must). The dual function pouch / shoulder strap can be pushed inside out to quickly stuff the bag back inside when you're done using it. Unlike other reusable bags that use elastic fabric to contain the bag, because the pouch is made of the same material as the bag, it keeps the stored bag nice and compact and won't expand over time.
Check out the Onya Weighs too. They contain 8 x mesh bags to put your produce (loose vegetables, fruits and seeds / nuts) in before weighing at the counter for paying. When you get home you won't need to take out the contents either. If the contents need washing before consuming, simply dunk the entire bag in water and a touch of Apple cider vinegar to wash, then pull out the entire bag - the water will just drain out. Store the contents in the bag in the fridge, and you'll find your fruits and vegetables will stay fresh longer too.