Platypus plastic water storage bottles are flat when empty.
They have a push-pull plastic nozzle on one end like the ones on the "sports" type water bottles. The nozzle unscrews for easy filling.
The Platypus bottles can be rolled up and stored in places where you can't fit a canteen. Great for your fanny pack or (large) coat pocket. Put one in your bugout bag or backpack as a spare in case you lose or break your canteen. They take up amazingly little room.
The Platypus one liter (34 oz) bottle only weighs 23 grams (.8 oz). The model name is "Platypus 1". They also come in other sizes. See www.platypushydration.com for more details. The manufacturer is www.cascadedesigns.com .
Platypus 1 bottles are available from South Summit at www.southsummit.com for $4.95 each plus shipping as of December 2003. The South Summit Item Number is CD07620.
I just bought some and they work as advertised. They are flat when empty, they don't leak, and they are reasonably tough. An unexpected advantage is that you can fill the Platypus bottles from shallow water sources like tinaja.
However, the Platypus water storage bottles do have some disadvantages. The Platypus bottles are not as durable as regular plastic or metal canteens. They will leak if abused. You can boil water in some metal canteens but not in the Platypus. Most canteen covers have a pocket for water treatment tablets, but the Platypus does not include any cover. So the Platypus bottles must be carried inside a backpack or other container along with water treatment tablets or a water filter.
The Platypus bottles are good as an emergency replacement or substitute for a canteen, but they don't entirely replace a canteen. So carry a canteen or sports bottle as your primary water storage device. Use the Platypus bottles when the bigger choices aren't available or won't fit.