As you probably know, a cameo in film is a brief appearance by someone who's well known, often instantly recognizable, to audiences at the time the movie is released. Usually the famous person plays him/herself, but not always. Marshall McLuhan, a communication/media theorist who reached his popularity in the 1960s and 1970s, was familiar to audiences who would have seen the 1977 comedy Annie Hall when it first played in theaters, although many viewers might not know him now. Fortunately, his popularity doesn't affect the punchline of the very funny scene he's in.
Alvy Singer (Woody Allen) and Annie Hall (Diane Keaton) are in a long line for a movie, trying not to fight about their sex life, and Alvy is continually annoyed by a guy in line behind him, who rants on pompously and erroneously about any number of topics. And eventually, Alvy is able to do the one thing we'd love to do when we encounter That Guy In Line. How is McLuhan involved? Watch the relevant part of the scene after the jump (YouTube link) and you'll understand why this cameo is still classic comedy, even though McLuhan is no longer as generally well known as he was.
Annie Hall is chock-full of familiar faces to modern audiences: cameos from Paul Simon and Dick Cavett, and minor performances from actors who became famous later -- Jeff Goldblum's notorious "I forgot my mantra" line, Christopher Walken as Annie's creepy brother and the blink-and-you'll miss it shot of Sigourney Weaver in one of the final scenes. However, these early performances don't count as cameos, since they weren't well-known back in 1977. Even if you don't get all the pop-culture references of the time, it's still one of Woody Allen's funniest films.