Jack Larson, Jimmy Olson on the 1950s Superman TV series, made a small appearance in Lois & Clark - The Adventures of Superman. In the story Jimmy Olson was made older and the older Jimmy was played by Jack Larson.
Tom Selleck in Coma Ted Knight in Psycho Judd Hirsh in Serpico Carl Alfalfa Switzer in Its a Wonderful Life (Is this a Cameo?) Stephen Spielberg in The Blues Brothers
Michael Palin as a member of The Royal Society for the Prevention of Cruelty to Children (RSPCC) in The Witches (1990)z
Only Spielberg would be a cameo. Switzer was just an older previous child actor with a small part in a movie. Selleck, Knight, and Hirsch were pre-fame actors also just doing a job.
Switzer also acted in The Defiant Ones. As jjh1959 mentioned, Switzer’s celebrity had fallen after childhood, and he was just a working actor. Sadly, his career was back on the upswing (John Wayne’s production company cast him in several films) when he died.
Jack Larson also appeared as a bartender in[The Return Of Superman]..Cuba Gooding Jr.-Coming To America.. James Drury as a young doctor in an early Sidney Poitier film[1950's].
Jake LaMotta in The Hustler. Kirk Douglas, Burt Lancaster, Tony Curtis, Robert Mitchum and Frank Sinatra in The List of Adrian Messenger.
Billy Crystal ( with a then unknown Dana Carvey) in Spinal Tap. Featuring the immortal line "C'mon, let's go. Mime is money!"
Longtime X-men writer Chris Claremont had bit parts in several of the films. Jonathan Frid in the Johnny Depp Dark Shadows
Just watched The Blues Brothers, and there was Pee Wee. Doubt if I even knew who he was when that movie came out.
Seems to remain an unresolved claim that Jack Benny's in "Casablanca", but this page has some info/pics: Jack in Casablanca Revisited
I first became aware of Paul Reubens as "Howie Hamburger Dude" in "Cheech and Chong's Nice Dreams". Not a cameo back then, though, since Reubens was just an unknown actor and not famous yet:
Reubens' began to have success with the Pee-wee character in the early 80s when he did the character as the focus of a live show in LA. That got adapted as a cable special, and he also started to play Pee-wee on Letterman. Eventually that led to "PWBA" and "Playhouse". So anything pre-1982 or so is just "Reubens acting" not "Pee-wee cameo". I forgot Reubens was in "Cheech and Chong's Next Movie" pre-"Nice Dreams". He played Pee-wee here, but a less childlike version than we're used to:
