| The first US late night network TV show, begun by NBC in the early
50s, thanks to Pat Weaver (Sigourney's dad). Jay Leno is the host
now, the chair once held by Steve Allen, Jack Paar, and Johnny Carson --
there were also many "guest hosts" (i.e. substitute hosts) over the years, prior to the advent of rerunning old shows, circa the late-60s.
Each host retweaked the mix of the same ingredients: sketch comedy, chat, an opening monologue, stand-up comedians, and musical guests. There were probably more instances, in earlier eras, of showbiz guests being on the show simply because they were entertaining, rather than the almost-100% certainty nowadays that a guest is on the show to promote some current piece of "product" (film, sitcom, CD, tour, centerfold, etc).
The viewership is small (compared to prime time), but the time slot became lucrative, resulting in many wars, feuds, and power struggles over the decades (see, for a latter-day version, The Late Shift). And it also resulted in SNL, and in the creation of late-late-night network TV, via Tom Snyder and David Letterman, rescuing us, for better or worse, from those Ronco ads, old movies, and the inevitable test pattern once Carson signed off for the night at 1 AM. | 
