Here's one I've never heard of.  On GunsAmerica, a Ruger Hawkeye in .256 magnum, selling for $1,995.

A single-shot pistol built on a revolver (Blackhawk?) frame, apparently.  According to the seller, the .256 is a .357 magnum necked down to .25 caliber.

Interesting.  Follow the link to see a picture of it.