It has become a bit of an obsession with me to see just how light I can go with my bag. Part of this drive is from necessity: I have an old back injury that is bothering me more so I need to minimize the strain on it. I don't want to wheel a bag, though, so I use a lightweight backpack with a decent waist belt to shift as much weight as possible to my hips. I try to keep the bag small enough so that it can fit under the seat (my current bag is actually made for runners to carry a hydration pack in).
Right now everything weighs a little less than 8 pounds, including the bag, but I think I can do better. I've toyed with the idea of taking just one pair of pants, rather than two, for example. My wardrobe is very bland, but boy is it liberating not to worry about finding a place to check luggage when I arrive somewhere.