The Internet's greatest Bay-Area stoner humorist, Seanbaby, offers his list of The Ten Greatest Internet Fads. I actually knew Jay Maynard, online anyway. He was a huge, I mean huge fan of "Animaniacs."
To Seanbaby's list I would add the phenomenon of Mr. T webcomics, which was a transatlantic phenomenon (there were probably more of them in England, where The A-Team has always been hugely popular, than in America). I previously wrote about the Mr. T craze here so I won't repeat myself.
Personally I think that the golden age of the Internet fad is over. People are more and more doing blogs instead of posting on newsgroups where they can obsessively discuss things; online content is more and more about expressing your thoughts rather than posting sound bites from obscure video games. I may be wrong, but I think there's no longer enough of an online "community" to make a fad; that's a relic of a time when people who posted on the Internet were like an underground cult.