There are also single-strap packs that are essentially a hybrid between a backpack and a messenger bag.
Inexpensive, very simple packs that combine the drawstring and straps into a single piece of cloth or webbing are occasionally sold for use at sporting events and the like. Some high-end retailers (notably Apple Stores) use a similar design for their shopping bags as well.
Rolling backpacks are backpacks with wheels on the bottom and an extending handle. Because of their design, rolling backpacks reduce the strain on the user, though the shoulder straps may be used to carry the pack for short distances when the terrain is not suitable for wheels. Rolling backpacks are most commonly used while traveling by airplane or train.