Tom Hanks played Uncle Ned in three episodes of “Family Ties.” In one of these episodes, he is revealed to be an alcoholic. It is an impressive performance.
Sylvester Stallone appears in one scene in “The Prisoner of Second Avenue.” He gets attacked (successfully!) by Jack Lemmon, who thinks Stallone has picked his pocket.
Lee Van Cleef is the soldier who fires the radioactive rifle grenade at the end of “The Beast From 20,000 Fathoms.”
:D. Search kept timing out, otherwise I would have included the link…
One more: The hooker hired to blow Charlie Sheen in the Limo at the beginning of Wall Street was none other than Andrea Thompson, she of NYPD Blue fame, who later had a brief stint as an anchorwoman for CNN…
Before he played Superman, George Reeves had a number of movie roles. I think the only one of these I have seen is in “Gone with the Wind.”
During the run of “Adventures of Superman,” he also had a small part in “From Here to Eternity.” After seeing him so many times in the Superman costume and Clark Kent’s suit, it is strange to see him in an Hawaiian shirt.
He ran the crooked Faro table in the Oriental, and gets slapped around by Wyatt Earp.
I enjoy watching credits from 70’s and 80’s movies to see if I can recognize famous actors when they were just starting out. However, now that the credits go so fast and are compressed, it’s quite hard to do.
Marilyn Monroe is seen in a canoe on a lake in Scudda Hoo! Scudda Hay! (1948), and as one of the chorus girls in A Ticket to Tomahawk (1950).
Clark Gable is a ballroom extra in The Merry Widow (1925).
James Dean is a youth who orders a chocolate malt at a soda fountain in Has Anybody Seen My Gal? (1952).
The old Perry Mason show is great at spotting future famous actors. Like a very young Robert Redford giving testimony in “The Case of the Treacherous Toupee,” looks like his 7th TV role.
Then there was an eternally young Dick Clark, who it turns out, committed the murder (“Yes! Yes I did it! And I’m glad!”). I guess by 1966 he was already a familiar figure with Bandstand, but it was still weird to be watching reruns and suddenly see him pop up.
And Gabriel Byrne played the Duke of Cornwall.
The part was originally much larger, but they cut it after preview audiences were distracted by seeing Superman in another role. It effectively ended his film career. So sayeth Biography on A&E.
When Benjamin & Elaine are in the hotel, watch for the first man who calls him Mr. Whatever (the name he registers at the hotel for trysts with her mother). It’s Mike Farrell.
I once saw a very old episode of Andy Griffith with Jack Nickelson in it.
Debra Jo Rupp (That 70’s Show) played Tom Hank’s secretary in ** Big [/b}, Jerry Seinfeld’s manager, and Lisa Kudrow’s sister-in-law on ** Friends **. (Phoebe was a surrogate mother for her).
MAS*H is a goldmine for seeing actors before they were stars. In watching the old re-runs, I have seen Ron Howard, George Wendt, John Ritter, Patrick Swayzee, and Shelly Long to name a few. It’s always fun spotting them with the double take that inevitably accompanies it.
“…none other than Andrea Thompson, she of NYPD Blue fame…”
I thought she was of Babylon 5 fame…which shows where MY loyalties lie. 
In “Coming to America,” Samuel L Jackson is the robber who tries to rip off McDowell’s. And apparently the little kid in the barber chair is Cuba Gooding, Jr.
Nicolas Cage is one of Judge Reinhold’s character’s buddies in “Fast Times at Ridgemont High.”
And I used to love “Quarterback Princess” starring…Helen Hunt. I was such a geek. (“was?”)
Gary Cooper has a bit part as a reporter in the silent Clara Bow film, It.
A very young Benicio del Toro played a knife-wielding henchman in License To Kill, the last Timothy Dalton Bond movie.
Ron Howard was a star long before MAS*H. A ltitle thing called The Andy Griffith Show (1960-68)
Leonard Nimoy has a tiny part in Them!
Forrest Whittaker is the football player whose car gets trashed in Fast Times at Ridgemont High.
Boy was I surprised to see the most excellent Laurence Fishburne in the excerable Cherry 2000.
Samuel L Jackson played the DJ in Do the Right Thing. I guess that’s not a bit part, but I didn’t realize it was him for a long long long time. And he’s credited as “Sam Jackson.”
Jeff Goldblum is one of the hoodlums who kill Charles Bronson’s wife in Death Wish.
Oh, since you mentioned Death Wish, I believe Marina “Deanna Troi” Sirtis plays a rape victim in Death Wish III. And as long as we’re mentioning Star Trek actors, check out Denise Crosby as the bat-wielding girlfriend in 48 Hours.