Zoolander. YES there’s a bazillion cameos. And I’m sure everyone knows Vince Vaughn (with no dialogue) is Derek’s brother. But the OTHER brother? Judah Friedlander!!
Jack Black is in the background of an Arrested Development episode. I think he’s playing darts in a bar. No lines and he does not face the camera or anything.
You would never know Daniel Craig was in The Force Awakens except for publicity about his cameo; he played a storm trooper and never removed his helmet.
Annie Hall, even aside from the small parts that you’ll recognize (Carol Kane, Paul Simon, Shelley Duvall, Janet Margolin, Colleen Dewhurst, Christopher Walken, Marshall McLuhan, and Dick Cavett), has lots of brief cameos from people like Mark Lenard, John Glover, Jeff Goldblum, Shelley Hack, Beverly D’Angelo, Tracey Walter, Sigourney Weaver, and Truman Capote (in the weirdest role, where he plays a Truman Capote lookalike):
Note that he's visible in that background of all of that video.In Young Frankenstein Mel Brooks had to Shemp* for Gene Hackman during the close-up of the soup being poured on Peter Boyle’s lap (reshoots were required after Hackman had finished his time on set). That’s Brooks’ hand holding the ladle not Hackman.
*I think many here know what Shemping is, but just in case.
In the movie Head, the feature film vehicle for The Monkees, there’s a blink-and-you-miss-it glimpse of both Jack Nicholson and Dennis Hopper.
There are many other cameos (Annette Funicello, Frank Zappa, Sonny Liston, and others) but these other cameos are much more obvious than the Nicholson and Hopper cameos.
It’s been publicized, but I doubt anyone realized that Cate Blachett was in Hot Fuzz by watching the film. (Peter Jackson also appears.)
During the Pterosaur attack in Jurassic World, Jimmy Buffett can be seen getting up from a table grabbing 2 margaritas, one in each hand, and running.
YouTube clip is only 49 seconds.
In Babylon 5, Scott Adams has a cameo as a man named Mr. Adams who thinks his cat and dog are trying to take over the galaxy.
The 3 Stooges: after Curly was replaced due to his stroke, he appeared as a train passenger in a subsequent film.
JB is also one of the blond, sunglassed agents in the fight at the end of Repo Man.
I can never make up my mind whether this was brilliant or stoopid.
Actors in nearly all major “alien” roles in B5 had other roles as humans. (ETA: Not the primary cast roles, but many continuing ones as well as one-ep appearance.)
There is one close to the end of “The Blues Brothers”. No, I’m not talking about Steven Spielberg’s cameo as the guy in the Cook County Assessor’s office. Everybody knows that one. It’s in the “Jailhouse Rock” scene in prison. The prisoner who jumps up on the table and starts dancing? That’s Joe Walsh.
In The Big Chill, Kevin Costner is Alex (the guy who died). But al his scenes were cut except a few shots of his (supposed to be) dead body at the mortician’s.
We need a little exposition in SUPERMAN THE MOVIE because, hey, if the plane’s not in radio contact with a drawling American saying stuff like “niner niner”, how are we going to hear that it’s Air Force One right before there’s engine trouble and our hero shows up and saves the day?
The voice on the radio is – Christopher Reeve.
There’s a movie called Won Ton Ton: The Dog Who saved Hollywood that had dozens of famous movie stars making cameo appearances. It’s just a silly B movie but it’s neat to watch it and see how many people you can recognize. According to IMDB Johnny Weissmuller was in that movie, he was the only one I didn’t recognize.
Not exactly a cameo, but a hilarious bit of “stunt casting”. On an episode of the old TV show “The Adventures of Brisco County Jr”, the title character was on a stagecoach with other passengers, one of whom was a botany professor in search of “medicinal herbs”. The professor was played by Timothy Leary.
*Frasier *used a huge number of famous voices for the call-ins. Some were just bland, but quite a few delivered great punchlines if you knew who “Fred” or “Wilma” really was.
One I recall right off hand is David Duchovny calling in to complain about his relationship issues of six years.
Ron Jeremy had a cameo in both Ghostbusters and an episode of News radio where Jimmy James is reading his translated-to-Japanese-and-then-back-to-English memoir in a bookstore.
Huey Lewis in Back to the Future – I had no idea that was him when I fist saw it.
Oscar the Grouch in The Muppet Movie. 
I’ve got two, one visual, one audio.
In Ant-Man when Rudd’s Ant-Man crashes on top of the cab, the cab driver inside is Garrett Morris who was the first actor to play the Ant-Man character. He did it on a superhero filled sketch in a 1979 episode of Saturday Night Live (hosted by Margot Kidder)
In the film Kingpin during the big bowling tournament, as one of the bowlers is about to bowl, an unseen person in the stands yells “Attaboy, Luther!”. Everytime Don Knotts character speaks in public in The Ghost and Mr. Chicken (1966), an unseen person also yells “Attaboy, Luther!”.