I also use the Camelbak Nalgene bottle/hose combo Steve mentioned earlier if I'm running my lightweight belt rig, and it's really nice too. If I'm running some old-school impression surplus canteens are great. I just keep a gallon jug at spawn to top it off since it doesn't hold as much as my MSR bladder.
Word of advice: get a bite tube cover if you go the hydration bladder route if you don't find a good place to stow the end of your tube. Low-crawling through the muck then drinking from a filthy bite tube is not fun.
