"This is a 9 litre (for you Imperials 1 litre = 32 oz., large size Nalgene bottle) water bag from Mountain Equipment Co-op. I have used it a lot this year and it is SO handy. It is one of my favourite pieces of gear!
It folds up very small when there is no water in it. It does not close so it is not meant for taking water while hiking; rather it is for around camp. It has a large hole in the top, and when you fill it up you just place it on a flat spot, and almost magically no water will spill out. Because of its construction, the water level will always be at the top of the bag no matter how full it is. (If you can't find a flat spot that's okay too, it just won't be able to hold all 9 litres.)
When camping, I used to have to make several trips to the water source filling up all my bottles and pots. Now I just make one trip.
The bag costs about $10 at MEC (which does mail orders for anyone not near one of the stores)."