I was surprised today to find that my lanyard string backpack (the one where the cinch cord for the top are the straps) worked great for a short commute where I didn't need to carry much other than a netbook. Lightweight, compact, and looks like a shopping bag when you need to duck into stores. If you get the type that is mesh, it aerates as well!

I'm sold - these are great for lightweight commutes. (My main commuter bag is a true Banjo Bros Commuter backpack, and for heavy loads, Jandd panniers.)

I commute with an REI Flash 18 - also a drawstring type pack - when I don't need to carry much. I drop the same bag into my Banjo Bros Commuter pack when I need to add more stuff. My poor man's modular backpack system. I have even taken the REI bag on long rides in lieu of my Camelbak.