At the 2012 Shot Show, Heizer Firearms was introducing the smallest .45 caliber pistol that money can buy. It's 5/8ths of an inch wide and weighs less than a pound.
Here a Heizer rep shows off the gun:
According to Heizer's website, the DoubleTap is the world's smallest and lightest .45 ACP concealed carry pistol on the market, and it has several patents pending. Two rounds can be stored in the gun's chamber with two additional bullets in the grip. Here's more info on the pistol from the press release:
The break-action features a patent-pending progressive barrel porting option that cuts muzzle rise and recoil. The ambidextrous thumb latch releases the action and automatically ejects the spent rounds.
Unlike any firearm, the DoubleTap features a truly revolutionary double-action trigger system that utilizes ball bearings for a smooth action and crisp trigger pull.
DoubleTap offers more specs in the video below:
The aluminum and titanium versions start at $499 and $729 respectively. It also comes in a 9MM version.
This story has been updated for clarity.