I have about a 7 mile commute.
In my messenger bag (I guess that makes me a poseur), I carry clothes, lunch, keys, wallet. If the weather is variable, I also carry knee warmers, tights, a windproof jacket and a polypro shirt.
On my bike in an under seat bag I carry tire levers, a patch kit, a tube, and some allen wrenches. Elsewhere on my biek I carry a pump and some water.