Lightweight water storage
I had an idea last night, Trader Joes sells an Australian wine that comes in a 4 liter box and has a spout w/ a twist valve on it.
I finished one last night and pulled the bag out to look at it. The spout just pulls out and pops back in and the bag is obviously designed to hold wine for months, so it's definitely safe for water.
It makes a ~1 gallon water storage bag w/ a spout that collapses as you use it.
When I head up into the mountains for camping or anything, I take bottled water along for emergencies. This will allow me to take water from home instead.
It could also be carried empty into the backcountry and then used to store water that you collect there.