I was going to mention **Adam Goldberg’s **appearance on “Friends” as Chandler’s crazy roommate that moves in when Joey moves out. It looks like he had a 3-episode arc though, but if we can count Slow Donnie on “JSM” than we can certainly include Crazy Eddie.
Charles Durning as a Medal of Honor winner on NCIS.
And just about all of the guest stars on Ricky Gervais’s Extras. Most notably Patrick Stewart and Daniel Radcliffe.
Scorpio is great.
My favorite one-off on the Simpsons has to be the dentist though. “Why must you turn my office into a house of lies?”
Fran Drescher as Bobbi Fleckman in The Nanny (shall I explain?)
Rachel York as Randy Wolfe in Diagnosis Murder

He was a mime, y’know like this… (she does the glass room thing) Only he was dead so it was more like this… (does the poked out tongue, hanging from a noose pose)
Sarah Silverman in ST Voyager she’s just disqualified from Monk though IIRC she makes a tiny appearance in Mr Monk’s 100th case
Another Voyager one, that guy who always plays psychos* plays a psycho- who comes good in the end. (Should I put that in a spoiler? Nah)
*Brad Douriff
I was going to mention **Adam Goldberg’s **appearance on “Friends” as Chandler’s crazy roommate that moves in when Joey moves out. It looks like he had a 3-episode arc though, but if we can count Slow Donnie on “JSM” than we can certainly include Crazy Eddie.
Oh, Crazy Eddie. He was hysterical. I love when he stuck a little goldfish cracker into the fish tank.
Phil Hartman on “3rd Rock From The Sun.” OH, YEAH!
ETA: Anf I can’t believe it took 86 posts to mention this, Phil’s last appearance.

Vincent D’Onofrio on Homicide: Life on the Streets as the homicide victim they met after he was killed but before he was dead.
Was he the guy trapped between the train and the tracks or something?
John Ritter’s appearance in first-season Buffy was pretty memorable, as the ultimate mom’s-bad-boyfriend.
Robin Williams on SVU
ETA: Tom Wolpat on Smallville, reuniting with John Schneider. Yeee-Haw!
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Actually, his second appearance was as the same character. On his blind date with Rachel he talked about losing his restaurant because of his drug problem. Monica had set them up. I guess they stayed in touch after the “tartlet” incident?
Oh wow…I guess I had forgotten that.
That sucks because I liked his first, not so much the second.
(I have such TINY FINGERS!!)

He appeared more than once, so he doesn’t count for this thread, but I only have to give the name:
Gandolph Fitch
I have no idea who that is. I don’t think I’m the only one.
From my childhood-
Mask and Shokoti from the two-parter in the original He-Man show. Creepier than most of the other episodes and I liked Mask a lot-would have bought his action figure.
Pity Brendan Frasier from Scrubs is disqualified, he was so good.

BrotherCadfael:
Well, sure, if you want to consider the animated series as canon… :rolleyes:
For those of us who don’t, the first command-level female character shown is the captain of the USS Saratoga NCC-1867 in the opening scenes of Star Trek IV: The Voyage Home, released 1986. There was also Saavik’s Kobayashi Maru training scenario in Star Trek II: The Wrath of Khan, released 1982.
It’s silly to count Saavik. She was not in command of anything; it was a training exercise, part of her academy training.
Tom Hanks on Family Ties. (Two part episodes count as one-off, and I don’t remember the drinking problem episode, so that doesn’t count either. )
Johnny, tell me what I’ve won.
Joan Sanderson as Mrs Richards in the Communication Problems episode of Fawlty Towers. The Wiki article on Sanderson says,
However she is probably best known to many for a single guest role: that of the abrasive and selectively deaf Mrs. Richards
WKRP had a bunch of these, aside from the aforementioned Scum of the Earth. Little Ed the Preacher, Hoyt Axton, Jennifer’s perfect boyfriend “Steele,” Bobby Boogie, Del from Del’s Stereo and Sound, etc.
The cranky old deaf woman in a Fawlty Towers episode. “What?” “Que.” “K?” “Si.” “KC? Whatever are you talking about?”
Also, Lawrence Tierney as Elaine Benes’ father.

WKRP had a bunch of these, aside from the aforementioned Scum of the Earth. Little Ed the Preacher, Hoyt Axton, Jennifer’s perfect boyfriend “Steele,” Bobby Boogie, Del from Del’s Stereo and Sound, etc.
One I would add is Bill McLean as Harvey Moorhouse the Barber, Les Nesmann’s biological father.
Bill McLean is one of those actors you immediately recognize and “know,” and I always think of him from that ep.
Oh, and Angelique Pettyjohn from “Star Trek.”

Telperion:
Indeed. Probably the best performance they ever had on that show, too.
The punchline, delivered by Shelley Long, is a personal favourite.
Care to share?