Geena Davis and Jean Smart guest starred on the same episode of Remington Steele.
Nicholas Cage as a counter employee in “Fast Times at Ridgemont High”.
Both not terribly obscure, but Johnny Depp in 21 Jump Street and Nightmare on Elm Street and Angelina Jolie in Hackers.
I just saw the Zac Efron episode two nights ago- but what I associate with a lot more is his appearance as Simon-as-a-child on Firefly. It was his first TV role, even.
To me, Courtney Cox was the girl in Masters of the Universe. 
My mental image of Hugh Laurie is one that is extremely clean shaven. One might say he was a bit of a star in the Black Adder days, but certainly not comparable to his current standard.
And I remember Soledad O’Brien as an anchor for the local NBC affiliate in San Francisco in the early '90s.
I was going to post that. I remember Geena Davis on “Buffalo Bill.” Without clicking your link, I also recall Max Wright, who later played Willie Tanner on “Alf” and Charles Robinson, who later became “Night Court’s” Mac.
Also from both Buffy and All My Children:
I remember Michelle Trachtenberg as Lily, autistic daughter of (someone I can’t remember).
I remember Jason Alexander in a mid '80s McDonald’s commercial. He was leading a crowd dancing in the streets talking about the new “hot side hot, cool side cool” burgers.
I remember Tom Hanks as the wimp who learned karate so he could come back and kick The Fonz’s ass on Happy Days.
I also remember Elizabeth Shue starring in all those commercials for Burger King.
Ooh, I remember Matt LeBlanc’s Heinz commercial too! I don’t know how old I was, but I thought he was so cute. When Friends first came on I was so excited because of him! I also remember Matthew Perry in some show where he went back in time and became hisself as a teen? No he died and went to hell and was sent back to right his own wrongs? Something, but his character went by Chazz as a teen! (Aha – it was called “Second Chances” and he was "too good for hell and too bad for heaven).
I remember Adam Sandler as one of Theo and Cockroach’s friends on The Cosby Show. I don’t know if I remembered it when he was on Lois & Clark or just realized it later in reruns, but Dean Cain played a racist bully on A Different World.
This is fun to think about! It’s hard to remember sometimes if I recalled them once they hit it big or if I saw the old bit part type of show later and recognized them.
Doesn’t everybody? Who actually watched* Buffalo Bill*, anyway.
David Boreanaz as Kelly’s date on Married w/ Children. I probably saw it in re-run tho, so can’t say if “I knew him when …”
Does Tom Hanks in “Bosom Buddies” count?
How about Keanu Reeves as the young teenage fuckup in Parenthood?
I once won a five dollar bet with somebody who refused to believe John Goodman was in The Big Easy.
I used to watch the old private eye series Switch back in the seventies (with Eddie Albert and Robert Wagner). I remember watching this episode which had this really beautiful woman in it. It was Jaclyn Smith, a year before she did Charlie’s Angels.
I believe they were from Yu Go Slavia.
You know, it’s harder to represent Spiner’s drawl in writing than I thought it would be.
Mary Tyler Moore had a LOT of bit players before they went on to megafame.
[ul]
[li]Henry Winkler - “Happy Days” (Rhoda’s Date)[/li][li]John Ritter - “Three’s Company” (married Ted and Georgette)[/li][li]Bernie Kopell - “The Love Boat” - (a blind date for Mary, set up by Rhoda’s mother)[/li][li]Mary Fran - “Newhart” (Mary’s anti-Semitic new friend)[/li][li]Fred Grandy - “Love Boat” (plays a reverend that wins a “Teddy Award”)[/li][li]Isabell Sanford - “The Jeffersons” (she played a mother to a co-worker Mary was doing a story on)[/li][li]Louise Lasser - “Mary Hartman” (played a banker that wouldn’t give Mary her money out of her bank without a substantial interest pentalty)[/li][li]MacKenzie Phillips - “One Day At A Time” [/li][li]Craig T. Nelson - “Coach” (a mechanic that wants to ask out Mary)[/li][li]Jeff Conaway - “Taxi” (a young man Lou’s ex-girlfriend is dating)[/li][li]Penny Marshall - “Laverne & Shirley” (Paula, a neighbor in Mary’s new apartment building, also earlier in the series she played someone in a single’s bar)[/li][li]Mary Kay Place - “Mary Hartman, Mary Hartman” (Paula’s roomate, they are nurses)[/li][/ul]
And I’m sure there are a lot more I missed.
Two other examples I recall were
Howard Hesseman playing a gay man on “the Bob Newhart Show,” and Jim Carey in “the Duck Factory.” THAT was a very funny show. I don’t know why it didn’t get picked up. Also Jennifer Anniston in “the Edge,” starring Julie Brown. She even showed great comic skill back then.
How about Radio?
When I was little, I remember my brother calling me into his room to listen to this “cool new DJ” named Howard Stern who was interviewing Peter Wolf from the J. Geils Band. We laughed at it because the DJ’s voice sounded goofy and the interview sounded fake—the interviewee sounded more like Wolfman Jack than anyone else (I have no clue how Peter W sounds, so it might have actually been him).
The station: WWWW “W-4” in Detroit, days before it became “All Country” format.
That’s the last I ever heard of the dude until I had grown up and moved to the NYC area and my co-workers were all talking about Howard Stern. When I heard the syndicated show, I was scratching my head, thinking “Is this guy the same doofus DJ I heard when I was a kid, or am I inventing memories?”
My questioned memories were validated when his “Private Parts” book and movie came out, mentioning the WWWW “All Country” format incident.
I remember Martin Henderson (from Bride & Prejudice) in th e1st episode of NZ soap Shortland St.
If he ever gets an overinflated ego, he should be forced to watch it again.
But he had made a name, at least among the cognoscenti, as “Siegfried” on Get Smart. And she had already made her mark as “Myrna” on The Odd Couple.
My own pick is Bruce Willis. I was quite impressed by his role in “Shatterday,” episode one, segment one, of The New Twilight Zone in 1985. I recognized him when Moonlighting and Die Hard appeared, but by then he’d lost The Method and had turned into just another bad, but appealing, actor. Contrary to its intent, Method Acting is most entertaining when the actor still believes that he has captured the real. They are so darned CUTE! in a patronizing manner.