I've been using an Ortlieb messenger bag for the last year for commuting and all my gettin' 'round stuff. I don't have a car and this has been my primary means of carrying groceries laundry and cycling neccesities.

It's a backpack style bag with all the straps you usually find on back packs for carrying a bunch of stuff.

What really sets it apart is the part of the bag that rests against your back is padded in a striped pattern for venting heat. Then on the inside of the bag is a flat plastic peice that acts as a stiffener to give the bag shape and to protect your back from pokey bits that maybe in the bag.

You could comfortably carry railroad spikes in this thing.

Ortlieb