The Tail Rider is one of Arkel's most popular bags.  It is aerodynamic and quite compact, but like a Tardis and can hold much more than it looks like it should.  It includes an integrated rain cover, padded handle for carrying if off the bike, a tab for mounting a rear flashing light, internal and external pockets, removable internal divider, expansion bellows, and d rings for attaching an optional shoulder strap (not included).