The brand escapes me at the moment, but the style of bladders used in my salomon running packs are excellent. They use a fold over top with a slider/clip thing to keep them closed. This is great because it allows the bladder to be opened completely which is handy for a ton of reasons: fast filling, easy filling from shallow pools, easy cleaning, steripen use, and mixing in Gatorade powder etc. the other end uses a valve that is opened via the male nozzle on a drinking hose, which I also like.
It's probably not the lightest system out there, but for me the functionality is worth more than the few gram weight penalty.
Edit: check out SOURCE water bladders. I think that's one of the brands.