Instead of stuffing clothes into your rucksack, the Rolo Travel Bag lets you keep your kit organized and dry. 9to5Toys Specials has it for $45 shipped.
This bag is essentially a roll-up wardrobe, with a waterproof nylon shell. Inside, you can pack your clothes into mesh compartments which are secured with zippers. There are two large pockets and two smaller ones, which are designed to keep folded garments as flat as possible. Once rolled up, the Rolo is secured with quick-release straps, and it is remarkably compact. You can either use the supplied shoulder strap, or strap it onto your backpack. When you need to access your clothes, you simply unwind the roll, and hang it by the integrated hook. This allows it to swing freely through 360 degrees, and the see-through pockets mean you can easily locate your clothes.
You can currently grab the Rolo at 25% off the standard price with free shipping to the Continental US.