I'm fond of this seat pack from Performance:
transit 50 wedge It holds it's shape when empty and sits upright at an attractive angle. Big enough to hold my essential emergency gear (tube, patch kit, multi tool)
Nashbar also has one that is very similar, which I have on my daughter's bike:
Detours Guppy
This one is also very similar is design and I wouldn't be surprised if they are the manufacturer of the Performance & Nashbar items.
timbuk2 seat pack