What thing (other than the obvious: camera) do you need with you when you go for a "typical" photo shoot or walk around? It's a "have to have" item. For me... it's a "Crumpler, Four Million Dollar Bag". This little camera bag is small enough not to get in the way but big enough to carry a 70-200 2.8 and a flash while I have the camera and a short lens out. If I forget this bag, like I did today <grrr>, I feel unprepared. I had to carry a second lens in the pocket of my hoodie I was wearing.