Lee Goldberg is a producer and writer who is (in my opinion) one of the top televisiologists in the country; right up there with David Bianculli. (Not that I always agree with either one of them.)
Goldberg has noted on his blog today a recent addition to those TV cliches that may be annoying to some, but which form the basis of the Television Mythology.
[THE INVISIBLE CHARACTERS]
There you'll find his observations on the recent glut of shows, like 'The Ghost Whisperer', 'Medium', 'Monk', and 'Nip/Tuck', in which characters talk to people who aren't really there. This formula has really gained popularity since 'Six Feet Under' first aired, but shows have been doing it for years.
'Rescue Me' is another current biggie for this new TV cliche.
Some others I can think of - Bess Armstrong as Maxwell's dead wife on 'The Nanny', Elliot Axelrod appeared to Fiscus (at least in dreams) on 'St. Elsewhere', and Gary visited Michael for a few episodes after his death on 'thirtySOMETHING'.
But I don't know if I'd include 'Ghost Whisperer' or 'Medium' in this category, as those really are supposed to be ghosts and not just figments of the imagination, as all the others are. ('Shades of L.A.' had a similar kind of premise about ten years ago. Probably plenty of others.)