How about Xena Warrior Princess where Xena debuted in Hercules The Legendary Journies?
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Or how about The Lone Gunmen from The X-Files? That might have been a spin off.
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NCIS has been mentioned but the arguably more popular CSI introduced all the sister shows with back door pilots.
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The last season of Highlander had episodes where the title character didn’t appear at all while they tried out ideas for a spin off series.
Most of the season was that way. It was terrible.
I saw a movie called “Backdoor Pilots.” but that’s probably a different thread. 
Speaking of Drescher, her show *The Nanny *had an episode that revolved around Fran’s favorite beauty salon. Not much Drescher, but a lot of Tracy Nelson.
The Diff’rent Strokes family visited Mrs. Garrett at the Facts of Life school, but I don’t recall which show’s episode that was.
You owe me a new keyboard.
There was one with Huggy Bear and some generic white detective that producers of Starskey and Hutch were looking to spin off into their own show.
It wasn’t quite a backdoor pilot, but they introduced that PI character on House with the intention of spinning him off into his own show.
The last several episodes of The Practice served as pilots for two possible spin-offs. One made it to series, Boston Legal. The other would have followed two of the charaters as they started a legal aid type practice. The concept seemed to be to have the two new shows highlight the differences between the high priced legal firms used by the wealthy versus the legal services available to the poor.
Stargate SG-1/Atlantis?
The majority of the final season of Highlander was a series of auditions for the female lead of the spinoff. In the end, they punted on all of them and went with an established character from earlier in the series to spin-off for Highlander:The Raven
Yep. They had a 3 part SG-1 arc that pretty much centered around Atlantis and that introduced Sheppard, Ford, Beckett and Tayla, IIRC. McKay and Weir were already known by that point, having been introduced in earlier SG-1 episodes.
Those played more like cross-over episodes than backdoor pilots. The “backdoor” aspect of a backdoor pilot is that you tune in thinking you’re going to see a normal episode of your favorite show, and you see something else entirely. The CSI spin-off pilots were pretty well trumpeted, and you knew what you were getting; IIRC, they were both two-parters that involved characters from both shows in both episodes. I don’t what you’d call it: a “cross-over pilot”; a “front door pilot”?
Murder, She Wrote introduced a character named Harry McGraw played by Jerry Orbach, who appeared in several episodes, and who later got his own show called The Law and Harry McGraw. The MSW episode "The Skinny According to Nick Culhane is a backdoor pilot for TL&HM, supposedly, albeit, Jessica Fletcher still appears in the ep, and it plays much like a MSW ep., with the exception that Harry McGraw solves the case.
TL&HM lasted only one season, and was technically a failure, which is a good thing, as far as I’m concerned, because it left Orbach free to play Lenny Briscoe a few years later. Harry McGraw had a lot in common with Lenny Briscoe. Briscoe was a little more world weary, and McGraw was a little less successful, but they were both wise-cracking crime-solvers.
In another thread, I mentioned a Happy Days spinoff that, IIRC, was indeed a backdoor pilot. The episode is “Fonzie Meets Kat” with Kat being essentially a female version of Fonzie.
I remember that episode. I wonder if any of the umpteen foreign remakes bothered to remake it.
The Lone Gunman were longstanding secondary characters, so I don’t think they count. There was never an X-Files episode dedicated to establishing the setting or new characters for there spinoff, as far as I remember.
They did have a backdoor finale, though, if you want to call it that. After the spinoff had been canceled they brought some of the characters from it to the X-Files and dedicated an episode to resolving some loose ends with minimal involvement from the FBI. Millenium got similar finale treatment, as well.
Oh, yeah, when the Ropers got their own show, wasn’t the pilot an episode of Three’s Company? It shows the Ropers moving into their new neighborhood, and meeting the neighbors (one was played by Patty McCormack, former child actress of The Bad Seed). It wasn’t so jarring, I don’t think, because, the show focused on the Ropers from time to time.
I don’t really know, because the show was on after my bedtime during the school year, and I only got to see it in reruns, which were sort of hit and miss.