The UL Down Inner Jacket is as light as, well, the feathers that stuff it. No one likes to haul around items in their pack that never get used, but extra clothes are always a good idea.
retail: $129
www.montbell.com
Because this jacket is so small (stuff size is 4 by 6 inches) and so light (7 oz for a medium), you won't mind carting it around on a tour even if you never pull it out. The 725 fill power-down is encased in a gossamer 15 denier "Ballistic Airlight" nylon that's both 50% thinner and lighter than more common down fabrics. To save more weight, the front closes with snaps instead of a zipper. Granted, this jacket is not as warm as a full-blown down parka; but at less than half the weight and bulk, its warmth and convenience make it the perfect extra piece that can live unnoticed in the bottom of your pack, until you need it.