Khan sites generally feature Kirk screaming 'KHAAAAAAAAAAAN!' in humorous or unexpected situations.
The original site  pre-dates YTMND; it was found on the /contrib page of the original yourethemannowdog.com. However, its popularity carried over, and the 'fury scream' became a YTMND staple.
Khan! was the first major fad on YTMND, sparking hundreds upon hundreds of imitators, and setting the precedent for fad or spin-off sites of notable YTMNDs.
Star Trek II: The Wrath of Khan is the second feature film based on the popular Star Trek science fiction television series. It was released in 1982.
In the movie, Kirk challenges Khan to kill him and the following exchange ensues:
Khan: "I've done far worse than kill you. I've hurt you. And I wish to go on hurting you. I shall leave you as you left me, as you left her: marooned for all eternity in the center of a dead planet, buried alive. Buried alive."
Kirk: "KHAAAAAN! KHAAAAAAN!"