My Saturn VUE has this nifty thing in the back - you lift up a panel, and a few plastic pieces pop up to essentially make two, milk-crate sized boxes. They're great for holding grocery bags, gallons of milk, bags you don't want to spill over, etc. It's a neat design but can really only hold 2 gallon jugs of something, or two brown paper grocery bags. Recently, I've used it for:
1. Transporting small "critter keepers" with animals inside to the vet, for work.
2. Groceries, of course.
3. Taking home a potted plant.
4. Etc, etc, etc.
I think it would be neat to have the same system implemented, say, in front of the passenger seats in an SUV as well as in the trunk, to be able to hold more. Another good thing would be something that holds up plastic shopping bags more efficiently - maybe take the same kind of design, and add something to hook the handles on to, so those bags don't spill.
The photos below are from the Saturn website:
^Here, you can kind of see the crease in the back, that will fold up to make the back portion of "the holder"
^This is the device fully expanded.