The AR-15 or AK Pistol
About the only reason I can think for these to exist is because someone realized they could legally get away with it. (AWB state folks: you're SOL.) Sure, the muzzle velocity drops to 1700 FPS and the flash is akin to a star-shell, but...okay, I'm out of ideas. Anything this can do, a real rifle could do better. It looks like fun...I guess that qualifies.
The Mare's Leg
Continuing with the above theme. Six rounds of .45 Colt from a twelve-inch barrel, slower and bigger than a single-action Colt. Other than playing Steve McQueen (and, hey, who doesn't want to do that?), there's no reason for this gun to exist. Not to mention the whole rifle-reciever-loophole-thing.
The Ruger 10/22 Gatling Kit
It's a machinegun, but completely legal!* This is probably the only time you'll ever see a crew-served .22. For those of you with $10k burning a hole in your pocket (and who doesn't), you could go whole-hog and get an actual (repro) gatling.
Smith & Wesson 3566
Because 9x19mm +P+, 9x21mm, and .38 Super (and, post-facto, 9x23mm Winchester) apparently don't get 'er done, S&W conjured up the 9x21.5mm .356 TSW cartridge and stuffed it in the Performance Center 3566 handgun. About the only thing good about this cartridge is that all the components and dies are exactly the same as every other 9x(something), so unless you run out of wackybrass it won't sting too badly.
The Thunder Five
Predating The Judge by about a decade, the Thunder Five is a .410 snubbie revolver. The optional .45/70 cylinder looks like