If you're a serious tourer or a hardcore commuter, you need Banjo Brothers' Waterproof Quick-Release Handlebar Bag. It uses their ingenious bag-in-a-bag design, letting the outside liner take all the abuse while the interior compartment stays protected and dry. It attaches solidly to your bars via a quick-release mount with no other straps needed. A removable clear map pocket keeps you on track and you have quick access to gear with two mesh side pockets.
Capacity: 430 cubic inches