Outdoor Research Ultralight Dry Pack Liners

Backcountry.com says:
For extended backpacking trips in Alaska, the Pacific Northwest, or other soggy locales where a hydro-douching is all but guaranteed, throw the Outdoor Research Ultralight Dry Pack Liner in your backpack and keep all your stuff dry as a bone. This lightweight roll-top sack features seam-taped waterproof nylon with a durable buckle to keep out aqueous intrusions. After a steady downpour or a failed creek crossing, this pack liner will have a pair of dry undies waiting for you.