Originally posted by twilhelm What are others using and what's available in a similar size?
My favourite messenger bag is the Peak Design Everyday Messenger 15.
Close behind (and maybe equivalent if you don't use Peak Design's Capture Clip) are Think Tank's Signature 13 and Retrospective 30 (I like the Signature better). Mindshift Gear (Think Tank's sister company) also offers a messenger, which I have never tested but which has many interesting features, the Exposure 15.
Many other companies offer messenger-style bags. Lowepro, Manfrotto and Tenba come to mind. You'll get more basic designs, but good quality from these companies.