Does anybody know what this is? Somebody gave me this with their old pack.
I expect it's a camp tap. You fill it with water, hang it somewhere by the handle, and can get a little water from the wine-box like valve on it. It doesn't take up space when it's empty, unlike a bottle, and you don't need two hands to operate it, are less likely to overpour or spill, etc.
Here's one of the water storage bags REI sells these days: