The one thing I always carry extra of is water. It seems like every other time out I share water with someone who has run out.

I hate it because it is probably the heaviest thing I have to carry. Depending on where I am going and doing, I usually carry 2-4 liters of water and sometimes up to 6 if it is long and strenous. Some hikes just don't afford you a chance to replenish because there is no source of water on the trail.