Club Littlegun has photos of some very curious antique guns, including this pistol ring. I saw one of these for sale years ago at an antique shop in Aspen. The owner told me that they were popular among Nazis and the Mafia. I can't remember exactly how much it cost, but it was in the thousands. Club Littlegun also has photos of a belt gun, a watch gun, a crucifix pistol, a pen pistol, a hand cannon, and many other unique firearms.
Link (via Damn Data)