As a newbie bike commuter (my first week), I just bought this pack

Outdoor Research Dry Peak Bagger
http://www.mountaingear.com/webstore...FbFaMgodlCkAPA

I does not have any structure and hence is extremely light at 270 grams. (that's a little over 1/2 a pound)

Very comfy on my ride in this morning with a change of clothes, some food, spares, etc ...