I use a small Chrome Industries backpack -- unsure of the model, must have been discontinued because it's not on their website. May have bought it in their SF store on closeout...

Was looking for a smaller one so I wouldn't be tempted to load it down. Had tried messenger bags and they just weren't doing it for me. On other commuters, I've used panniers and front rack with handlebar bag. But my new commute is shorter and involves two bikes so I figured instead of outfitting both with hardware to take panniers or a front bag, I'd just switch to a backpack. Haven't regretted it yet...