I enjoyed - and was surprised by - the cameo by Patrick Swayze in “Donnie Darko”. He really fit, and must have enjoyed it. Another enjoyably surprising cameo was Christopher Walken as the Headless Horseman in “Sleepy Hollow”
Any favorites you all would care to mention?
Christian Slater as a walk-on ensign on Captain Sulu’s ship in Star Trek: Generations. Didn’t even take credit for it.
Kurt Vonnegut helping Rodney Dangerfield cheat in Back To School. Later, the English Professor flunks him: “I don’t know who you got to write your term paper, but he doesn’t know a thing about Vonnegut!”
Linda Blair [the Exorcist] in ‘Scream’
Charlie Sheen in ‘Being John Malkovitch’
and ofcourse Marcel Marceau in ‘Silent movie’ 
David Bowie in Zoolander!
I second that. And Robbie Coltrane was good in Frasier last episode - 1 (I havent’t seen the second part yet)
Dodgeball has three great surprising ones:
David Hasselhoff coaching the German dodgeball team, and
Lance Armstrong guilting Vince Vaughn into coming back to the team, and
Fucking Chuck Norris!
There were some great cameos at the beginning of Austin Powers: Goldmember.
A lot of people hated the movie but Sean Connery’s arrival as King Richard in the Kevin Costner “Robin Hood” completely redeemed it.
Willie Nelson in Half Baked–not exactly high art, but a hilarious cameo.
It was Star Trek VI.
I love Patrick Stewart’s arrival at the end of Robin Hood: Men in Tights. I also like Connery’s appearance at the end of Prince of Thieves.
I was thinking earlier today of the following cameo. It’s not a particularly good performance, but maybe that’s the point. What’s clever is that you had to watch both a movie and its sequel carefully to understand the joke. In the movie Scream, one character is telling Neve Campbell that if they ever make a movie about the events that are happening to them, her character should be played by “a young Meg Ryan.” Campbell replies, “Yeah, well, with my luck, my character will be played by Tori Spelling.” In Scream 2, they have in fact made a movie about the events in the first movie. You are only offhand told little bits about this movie within the movie. At one point on a TV in the background you see an interview with the actress who plays Neve Campbell’s character in the movie within the movie. It’s Tori Spelling.
I actually fell out of my chair laughing at Oliver Stone’s cameo in Dave.
I saw Dave. Where was Stone?
In the movie Dave, Kevin Kline plays both a President who has a stroke which incapacitates him and the actor who’s hired to pretend to be the President. (Yeah, it’s not very realistic.) Nobody notices the switch, and a lot of people like the actor better, since he comes across as a funnier, friendlier guy. However, you see The Larry King Show on TV at one point, and Oliver Stone is the guest. He’s the only person who’s noticed the switch. He’s trying to convince people that it’s not the President anymore, but everyone considers it just one more of his nutty conspiracy theories.
Stone was on TV saying that there was a conspiracy surrounding the president.
A few of my favorites:
Marshall McLuhan in Annie Hall (maybe the greatest of all time)
Mel Gibson in Father’s Day as a guy into body piercing
Brian Donlevy and Akim Tamiroff in The Miracle of Morgan’s Creek, reprising their roles from The Great McGinty
Bing Crosby in My Favorite Brunette
James Coburn in California Suite – very easy to miss – he’s Maggie Smith’s co-star in her movie within a movie.
Walter Huston in The Maltese Falcon – helping his son out in a nonspeaking part.
Joseph Cotton in Touch of Evil – showed up on the set and Welles put him in a couple of scenes.
Margaret Rutherford in The Alphabet Murders
Tom Petty in The Postman. (Movie was only so-so, but the cameo was entertaining.)
Didn’t Molly Ringwald do a recent cameo as a teacher in a teen movie? Didn’t see the movie, but that sounds like a great cameo.
Robert DeNiro as the commando repairman in Brazil.
She played a flight attendant at the end of Not Another Teen Movie who tries to talk sense into the male & female leads who fall into a cliched teen movie romance of the rich/popular guy with the poor/unpopular girl.
Some great lines from her part (Practically all her lines)
Background: Jock is trying to win back Nerdy Girl before she boards her plane.
Jock: some nonsense
Nerdy Girl: crushing on him
Molly Ringwald: What? That’s a line for She’s All That!
Nerdy Girl: Hey I shoulda known that. I masturbate to this movie.
everyone: weird looks
Nerdy Girl: embarrassed
Jock: to Molly Do you mind?
Molly: Not at all, I think masturbation is quite healthy.
later on
Jock: more nonsense
Nerdy Girl: still crushing
Molly: Wait a minute. That’s from Pretty in Pink!
Nerdy Girl: Are you sure?
Molly: Errr, yeah!
Lines are totally off the top of my head, and I haven’t used spoilers becuase if you are really that angry at being given spoilers of this movie then you need to get a life