The Western Flyer from Tom Bihn is a small duffel/backpack conversion travel bag designed for quick trips or minimal travelers.
Other Bags to Check Out:
The Blink & Wander packs from Timbuk2 are tough, rigid little numbers for travel with unfortunately weak feeling zippers.
The Aer Duffel Pack is not the tool for every job, but for particular uses it just might be perfect for you.
The Hexad Access Duffel from Wandrd is a highly organized bag with multiple large compartments that's great for anyone traveling with a serious camera setup.
The GR2 from GoRuck will last your entire life, features a segmented main compartment, is extensible with pouches and accessories and boasts a very rugged, handsome look.
At first glance it looked weird. Then I used it for a couple days. Good god does this bag do it all well. Seriously, so much organization, so well built.