Blackburn drink nipple, bought 1996. When I first bought a hydration bladder, it had a foul tasting rubber nipple thing, which was also quite difficult to get much flow from. Someone recommended the Blackburn nipple, which was quite expensive, about £7, but it has a spring-loaded valve which you push with teeth or tongue, and is then very free flowing. It used to be blue but has faded to a dull grey: