Sometimes it can be good to have an extra bag in your pack, for example if you have a large suitcase but want a bit smaller backpack for excursions. Freerain24, as the name suggests, is the backpack that holds 24 liters and the main compartment is waterproof. There is an additional quickly accessible non-waterproof zipper pocket. The bag itself is constructed out of high quality fabric while the straps are made of breathable material. When collapsed the Freerain24 packable backpack ($59.99) measures only 5 x 3 x 2.5 inch and weighs 157 grams.
Available at amazon.com for $59.99