It's a Mad Mad Mad Mad World had tons of cameos, among them:
Don Knotts
Jack Benny
The Three Stooges
Buster Keaton
Jim Backus
Ethel Merman in Airplane!
Peter Cushing in Top Secret
Martin Sheen in Hot Shots part Deux
Marcel Marceau in Silent Movie. (the famous mime has the only line in the whole movie)
Billy Crystal and Carol Kane in The Princess Bride
Charlton Heston in Tombstone
'Superman' on I Love Lucy. (I Love Lucy had a lot...like John Wayne, William Holden...)
My favorite Hitchcock cameo was in Lifeboat. The before and after weight loss ad.
Kurt Vonnegut in Back to School, along with Oingo Boingo.
Any appearance by Bruce Campbell.
The Police Squad tv show always opened with a 'special guest' like William Shatner or Lorne Greene....who was then killed.