My pack is made by Ultimate Direction - called the Sip Stream. I love the tank, it opens at the top. To close it you roll it down and fasten with velcro. I've never had a leak. They don't make the Sip Stream any more, but they have a variety of styles.

Ultimate Direction

REI also has a few of their own Novara brand, they use Nalgene (sp?) tanks, and have a very nifty magnet to hold the tube when you're not drinking.