I’m not talking about reunion movies but on other sitcoms.
It most often happens when the TV series they guest on is a spin-off; for example:
Woody Harrelson (Woody), Ted Danson (Sam), John Ratzenberger (Cliff), Rhea Perlman (Carla), and others reprised their Cheers characters several years later in guest spots on FRASIER. (I count Lilith as a recurring Frasier character.)
Katherine Helmond played Jessica Tate’s ghost (or spirit, or whatever) on an episode of Benson after SOAP ended and even gave a bit of closure to the cliffhanger that ended the series (revealing Jessica was in a coma in a South American hospital after her firing squad).
I vaguely remember the Kravitzes appearing on an episode of the sorta-kinda spinoff Tabitha. (I say sorta-kinda because even though it was a spin-off, they changed the character’s age and back-story from Bewitched).
But some have guest starred on completely different shows after their own have ended:
Jack Riley played patient Elliott Carlin on an episode of Newhart (where the crowd went wild of course, especially when his psychotherapist mentioned that the man’s mind had been messed up by a hack in Chicago.)
Bob Newhart himself played Bob Hartley on an episode of Murphy Brown on an episode where her secretarial problems had finally been solved by wunder-secretary Carol (Marcia Wallace) who also guest starred in character.
Speaking of Bewitched, Bernard Fox has appeared as Dr. Bombay on episodes of the soap opera Passions and Sabrina.
There was that Family Guy episode where they revealed that it was all a dream, and Patrick Duffy and Victoria Principal were standing in the shower together and slowly looked at the camera.
Besides playing Det. John Munch on “Homicide: Life On The Streets” and “Law and Order: SVU”, Richard Belzer has also played the character in cameos on several series, including “The X-Files” and “Arrested Development”.
Several cast members from The Brady Bunch appeared as their Brady characters in an episode of a show called Day by Day. Speaking of The Brady Bunch, Ann B Davis played a character named “Schultzy” in the first Brady Bunch movie, which is the same name as her character from The Bob Cummings Show.
The principal on Boston Public appeared as a defendant on Boston Legal. Betty White’s character from a three-parter on The Practice also appeared on Boston Legal and was popular enough that she appeared about a dozen times.
I’d list Shatner and Spader too but they’re the centerpieces of Boston Legal, not just cameo characters.
Let’s not forget Robert Patrick reprising his role as the T-1000 in Wayne’s World.
Just a nitpick, I usually think of a “cameo” as a short appearance thats usually incendental to the plot (and wikipedia appears to back me up). A lot of the Cheers characters that appeared on Fraiser had episodes based around them, I don’t think that they count as “cameos”.
Mine would be DeForest Kelly on the first episode of ST:TNG
On an episode of Scrubs, although unnamed, they had four of the doctors from *St. Elsewhere * playing doctors suffering from Legionnaire’s Disease and being treated by JD.
Jay Mohr as the character Peter Dragon from the brilliant but shortlived Fox show (is there any other kind?) Action! had a rather hysterical cameo on another show from what I understand. He showed up on a golf course, began one of his typical rants, and then promptly dropped dead of a heart attack. The doctor on the scene declares him dead and says, “Time of death: Thursdays at nine PM.” which was a jab at the time slot Fox put the show in.
gotcha