Ortlieb's Seatpost-Bag is an easy way to add extra carrying capacity to your bike. Tool-free mounts make it easy to get on and off, and switch from bike to bike. The roll-top closure makes it sealed from the weather and splashes, and allows for tight packing. The whole thing is made from waterproof nylon and Cordura, and even acts like a fender, keeping your back muck free. It's available in two sizes, the small one being suitable as additional bag for longer trips, the large version might even replace a backpack.