For the peeps who revere old covert weaponsSome neat-looking stuff here, though one wonders how functional it truly was.
http://www.darkroastedblend.com/2008/02/miniature-spy-guns.html

The 'suspender gun' they show is actually a sleeve gun, also known as a 'hold-out' pistol. I've seen a modern version of this that fires a pair of .22 rimfire shells. I considered it a curiosity, as I cannot imagine an officer missing it during a body frisk.

The 'pen gun' has many variants, I've personally seen two, and built one for a class in the Army. Most are .22 or .25 caliber, and usually rimfire. The ones I have seen were cocked by pulling the pocket clip out to about 75 degrees, then returned to the normal position. Firing was accomplished by pressing the button on the end, like on a ballpoint. You held it like a knife in an overhand stabbing grip, thumb toward you and pinky towards the target. Effective range was only about 10 foot, and it was intended for point blank (arm's reach) distance. In all cases, hollowpoint ammunition were used.

During WW2, the OSS (God bless Bill Donovan) created and used many such concealed weapons, including a Zippo lighter variant similar to the one displayed on that link.