Another Friends one would be when David Schwimmer as Ross showed up on the Single Guy. IIRC, Jonathan Silverman and David Schwimmer were really good friends in real life. Silverman also played a doctor on Friends for one episode also (delivered Carol’s baby maybe?). And the main character of The Single Guy had a best friend played by the same actress who played Carol’s life partner Susan on Friends.
There was also an episode of Friends with Caroline in the City on it, but I can’t remember if she was in character or not.
Yes, in character. The Friends/Single Guy/Caroline in the City/Mad About You crossovers were all part of a special event one night on NBC where characters from the shows showed up on other shows. At the time they were all part of the Thursday lineup.
Another fairly recent in character cameo after a show has ended would be Ray Romano reprising his role from Everybody Loves Raymond on The King Of Queens. He had made previous appearances on the show (and most of the other Raymond characters showed up a time or two as well) but the last one was the year after his own show had ended.
There was an episode of Caroline In The City with a cameo of Niles Crane & Daphne. Was that part of the same night? (Not quite what the OP is looking for, both shows were still running)
In a Halloween-themed episode of McLoud, McLoud is investigating a series of murders. Each victim has been drained of blood, and has two puncture wounds in her neck. One of the suspects is an elderly man, who, in his youth, was an actor famous for playing Dracula. The role was played by John Carradine.
Similarly, there was an episode of the old TV series Route 66 (“Lizard’s Leg and Owlet’s Wing”) that guest-starred Boris Karloff, Peter Lorre,and Lon Chaney, Jr. (!!) Better yet, Karloff and Chaney get made up as Frankenstein and the Wolf Man!
There was also very nearly an episode of Pickett Fences that would have had Mulder and Scully show up due to a UFO sighting in the town. Negotiations fell apart though when one of the networks balked at what would essentially be promoting a competitor’s product.
I remember the Skipper and Gilligan making a few appearances. The only one I can remember concretely was in that craptacular movie with Franki Avalon and Annette What’sHerFace, Back to the Beach, in which Gilligan is sitting in a bar telling these hot chicks this crazy story about being marooned on and island, and the Skipper rushes in and to tell them they’ve got a job and have to go. “It’s only a three hour tour!” and drags him off. But the whole movie was making fun of past movies (e.g. Annette What’sHerFace had nothing but jars of peanut butter in her cupboards, referencing her long-lasting TV commercial deal with Skippy.)
Something in my lizard brain vaguely remembers a Columbo visit on some other show, like Banacek, or Mrs. Columbo, or MacMillan and Wife, or something. (And I think it was after Columbo had ended.)
Oh, never mind. I remember what I was thinking of. In the Far Away, So Close and/or Wings of Desire movies Peter Falk plays himself (as an angle who became human, then became the actor, Peter Falk) and a character in the movie keeps playfully referring to him as “Detective”.
Conrad Bain and Gary Coleman appeared as Mr. Drummond and Arnold on (I believe the final episode of) Fresh Prince of BelAir; they were interested in buying the Banks mansion. Ultimately it sold to George and Louise Jefferson (which I don’t believe was their first in-character appearance on the show) who had toured it along with Florence.
There was an episode of Roseanne the other night in which June Lockhart, Barbara Billingsley, Alley Mills (The Wonder Years), Isabel Sanford, and a couple of other “classic TV moms” appeared in character to take Roseanne to task for her show. Ultimately they agreed that if they made the money Roseanne was paid they’d have agreed to have lesbian kisses and dysfunctional families in their shows also.
I thought John Hillerman played “Higgins” from Magnum P.I. on a couple other shows, but it looks like they were concurrent to Magnum. I could have sworn it was later.