If your budget dictates one camera bag for all occasions, make it this one. The top-loading holster bag comes with two padded straps that work in multiple configurations: as a chest pouch (with the straps crossed in back), as a shoulder bag (with one strap), and as a backpack (with two shoulder straps). The primary compartment is just big enough for a DSLR with a four-inch lens, but small enough to feel unobtrusive.
The zippered lid opens in reverse direction, which made for easy access while our tester wore it as a chest pouch, and the straps layered comfortably beneath her backpack, keeping the Yellowstone from bouncing or swinging. Three low-profile pockets conveniently hold a lens cloth and filters, and the heavyweight nylon fabrics resisted abrasion, even when we dragged it up ladders and across Maine granite. Bummer: The built-in rain cover doesn't work when wearing the straps (use a shower cap instead). $54; 1 lb. 10 oz. (bag and straps); m-rock.com