The Platypus PlusBottle is a soft, flexible water bottle that when emptied, takes up almost no space. It's perfect for anything from day hikes to plane rides.
If you're one of those people that likes to have water with you everywhere you go, you know how annoying traditional water bottles can be to carry around. The Platypus line of bottles (the PlusBottle being the best designed) collapses to almost nothing when it's empty — just flatten it out and roll it up. It's also lighter than your average bottle, and is quite inexpensive at only $US12 (on Amazon).
It isn't the only bottle of its type, but it does seem to be one of the most popular and the best designed, with a small loop on the end that you can use as a carabiner clip (for you hikers out there. Other brands include the Vapur Anti-Bottle (with which I have personal experience, and can vouch for) and the Evernew bottle.
The only downside of water bottles like this is that you really can't drink them one handed... or at least, it's a bit more difficult to do so. Still, I'd give that up in a second for something that's this easy to carry around. Hit the link to check out the Platypus line of bottles, or check out one of the links above to see its competitors.