OK, so it Is just a flexible water bottle. But it is (was) amongst the oldest items of gear I have been using continually on just about all of my backpacking trips over the past decade.
The Katadyn BeFree filter has been a favourite piece of gear for the past five years. Unsurprising as it is light, compact, easy to use and has been successful in preventing me from getting sick on trail
Three Points of the Compass 