Gilbert Gottfried tells what is considered the dirtiest joke ever told at the Friars' Club roast of Hugh Hefner, with reaction from other comedians.
From the Wikipedia article linked above, because funny:
"Rumors cited in this film suggest that Chevy Chase used to hold parties at which the goal was to tell the joke for an hour, without repeating any of the acts contained in its performance. Penn Jillette notes in the movie that no one has ever been able to listen to Chevy for an hour."
Edit: If you go to Youtube, there is a version by Bob Saget that's even dirtier.

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