Johnny Dark is a very funny stand-up comic who first appeared on TV in the '70s - about the same time as David Letterman, on whose CBS talk show he currently appears in a very funny recurring sketch.
On most Fridays, Johnny appears as Stanley Cronkite - a miserable employee at the CBS store on the corner of the building where Letterman tapes his show. They do the segments live from the real store which really does exist on the corner. If you've ever been there, you know they sell mostly crap - CSI keychains, Price is Right sweatshirts and the like.
And when Letterman says, "Craze, how 'bout a song?" - that's a parody of Jackie Gleason's "Joe the Bartender" sketch - which can be viewed here.
