.380 ACP
Chances are good that you've heard of this one, even if you're new to the wonderful world of shooting. The .380 is a small step up from the previous two, and is more widely available. A lot of shooters carry a small pocket pistol chambered in .380 as a backup for their EDC (every day carry). Depending upon the area where you live, a box of these little guys can be quite expensive, and is considered by many to be the absolute smallest self-defense cartridge you'd ever want to carry.