#5: Marshall McLuhan in Annie Hall
Probably one of the most famous film cameos for a writer, Marshall McLuhan's appearance in Annie Hall is still one of the great moments of film comedy.
#4: Francis Ford Coppola in Apocalypse Now
Coppola's appears briefly as a tv director during a battle scene.
#3: Hunter S. Thompson in Fear and Loathing in Las Vegas
Johnny Depp, as Thompson alter-ego Raoul Duke, drops enough acid to break down the fourth wall and see the real Thompson.
#2: Thomas Pynchon on The Simpsons
I was unfortunately unable to find any video of the appearance. As far as cameos go, this is one of the most bizarre. You have to remember that, until this point, there were no extant recordings of Pynchon. Literally zero recordings of one of the most notable living American writers. And then, in 2004, after nearly forty years of silence, he appears on the Simpsons.
#1: Kurt Vonnegut in Back to School
Back to School is a great 80s college movie starring Rodney Dangerfield and a before-he-was-famous Robert Downey Jr. It also has one of the best used cameos I've ever seen. (Also, I'm a huge Vonnegut fan, so I'm a little biased in my choice of #1.)