The Flaptop Bag
The multi-purpose ILE bag--Can hold cargo such as wheel boxes and bulky items, but then also has 1900 cubic inches of waterproof interior capacity. It has compression straps to cinch it all down when the load is light, and its narrow enough to always have over-the-shoulder visibility while riding in traffic--except maybe when you've got a huge Yakima box strapped on.
This one should go to Downtown from Behind!!!