Wesley Snipes played a gang member in Michael jackson’s “Bad” video.
Now doing time for tax evasion.
Nimoy also shows up in Them! along with soon-to-be-Davy-Crockett Fess Parker.
Clint Eastwood also shows up in Tarantula, but he’s hard to recognize.
My favorites are the two on the left from My Bodyguard and this thug in Woody Allen’s Bananas
Adam Baldwin, Matt Dillon and . . . well, you should recognize the third easily.
Yes, I remember it well.
Did I miss it? But has noone mentioned James Arness playing The Thing From Another World in the movie of the same name (1951)?
Not to mention Meathead. 
Also Teri Garr and Alex Rocco.
On old episodes of The Rockford Files have been Robert Hays, James Cromwell (oldest IMDb credit for each), Mary Frann, and Carlene Watkins (who would both go one to play wives of Bob Newhart on TV series).
Most of my favorites have been taken already, but there’s also:
Charles Bronson as the silent henchman to Vincent Price in House of Wax
Leonard Nimoy in Zombies of the Stratosphere
Leonard Nimoy and William Shatner both guest-starring in a pre-Star Trek episode of The Man from U.N.C.L.E.
Nimoy again as a reporter in the Outer Limits Episode I, Robot.
(Actually, lots of Star Trek alumni showed up in Twilight Zone and Outer Limits – including Shatnet several times, George Takei, and, I think, Jimmy Doohan)
A pre-All in the Family Sally Struthers was “The Smothers Brother Dancer” on The Smothers Brothers Show
A pre-Laugh-In Goldie Hawn played Ronnie Schell’s girlfriend on Good Morning World
A pre-Laverne and Shirley Penny Marshall was a semiregular on The Odd Couple
It still amazes me that Jim Carrey , long before his movie fame and pre-In Living Color actually starred in his own TV show, The Duck Factory, which was rapidly forgotten. Years later, when Comedy Central ran the movie Earth Girls are Easy on TV they didn’t even mrention Carrey’s name in the promos.
James Cromwell also played Archie Bunker’s friend, Stretch Cunningham on “All In the Family” around that same time.
I remember seeing John Travolta in commercials before he was in “Welcome Back, Kotter,” although I didn’t make the connection until much later. The same thing with Dustin Hoffman in those Volkswagen commercials in the late 60s.
Here’s one that amazes me that hasn’t been mentioned yet: Ted McGinley.
I think he’s played the “generic good-looking blonde surfer guy” for so long, he has become part of the scenery. He’s been a regular on Love Boat, Happy Days, and Married with Children, but his first major role was in Revenge of the Nerds.
He’s not that famous, but last night while watching “The Aviator,” I noticed that Matt Ross, aka Alby from Big Love, was in it. He played one of Howard Hughes’ engineers.
In retrospect, it seems that 1993’s Demolition Man had a pretty stellar cast. In addition to Sylvester Stallone and Wesley Snipes, there’s also Sandra Bullock, Dennis Leary and Jack Black, before any of them became really famous.
Don’t forget Benjamin Bratt, pre-Law and Order.
Watching Singles recently I realized that it had a fascinating cast and right down the bottom is Kissing Man played by Paul Giamatti.
I remember both - bits of the Duck Factory theme occasionally surface as an earworm in my head (“…but it sure beats workin’ for a living…”). Did Carrey have any actual lines in Earth Girls?
In other news: Keira Knightley appears as a non-speaking flute student in fart-joke-movie Thunderpants, which I believe came out around the same time as her big debut in Bend it Like Beckham.
And Krull viewers may spot the lurking of Liam Neeson in a minor role (as well as Todd Carty, for you EastEnders fans).
No this is all wrong Thunderpants was Paul Giamatti 10 years later but Krull was early Robbie Coltrane.
He wasn’t exactly obscure at the time, but given his later turn in The West Wing I love seeing Martin Sheen as the
crazy future President
in The Dead Zone.
I’m confused. What’s wrong about those? Neither Neeson nor Knightley were famous at the time of those appearances.
Coltrane and Neeson are both in Krull; Neeson has a few good lines and even a nice death scene.
Got quite a shock when I saw John Goodman and Jay Thomas in little throwaway roles as cops in C.H.U.D.. Goodman did it right after his role as the coach in Revenge of the Nerds.
Lawrence Fishburne, mentioned earlier, has a small role as a hospital orderly in Nightmare on Elm Street 3: Dream Warriors. The credits list him as “Larry Fishburne”.
A young, unknown Jack Nicholson was in a couple of episodes of The Andy Griffith Show, most notably playing the defendant in trial with Aunt Bee on the jury. He is even questioned on the witness stand, but he doesn’t tell anyone they can’t handle the truth, which is too bad.
I have to say, there’s something very incongruent about seeing Jack in Mayberry.
Michelle Williams as the young version of the alien in Species. You can spot her at 0:25 of the trailer.