There's always someone trying something new, something that will be useful in a very small number of situations. Or just something that will be neat.
Case in point is this .22 caliber knife. The knife has 4 small chambers in the handle for .22 caliber rounds. Twist part of the handle and a trigger and sight pops out (the trigger is sticking out the top of the knife while the primitive sights are below). Four shots are available before reloading.
Who could use this type of weapon? There might be a use for commando groups engaged in a night operation or beach assault. I wouldn't want to rely on it for regular self-defense work.
Another rather neato concept is one of a kind weapons built by a gunsmith. Mostly these are functioning guns built specifically for display, and some of them can look very striking.
Way back in the day these hand built guns were popular for people trying to compete in extreme long-range shooting. Since this type of long shots weren't possible unless someone steadied the gun on a bench, it became known as "bench rest shooting". The guns for this type of precision shooting were nothing more than the actions and barrels bolted down to heavy steel plates. At first these guns were known as "iron monsters", but now they are more commonly known as "rail guns".
Some firearms are made for deep concealment. Here's a one-shot pistol that looks like a pen. Scroll down to the bottom of the page and see the attachments that you can hook on to it to form a half-assed rifle (why not buy a real rifle?). Here's a pic of a pen gun that looks more like a pen.
During World War II some auto makers in the United States churned out a million of these Liberator one-shot pistols. They came with a little bubblegum cartoon showing a freedom fighter sneaking up behind Herr Nazi and shooting them in the head. Then the freedom fighter was supposed to steal Herr Nazi's gun and run for it. The idea was to air drop them in occupied Europe and watch the population revolt against their Nazi masters.
Interesting idea, doomed to failure. If you're going to risk a plane to air drop these little one-shot guns then you might as well drop real weapons. It only took about $1.25 to make each Liberator, but it only took $15.00 to make a cheap .45 sub-machine gun. Might as well drop the sub guns unstead.