One thing that amuses me when reading what people say on the internets is the attitude so many have towards what is popularly known as "fan fiction."
They talk about it like it's a new phenomenon, and the behaviour of the people who write it is hopelessly childish.
Now, I can understand that attitude, with some of the dreck I too have read (although I've stayed away from it lately), but treating it as a new trend. The "Mary Sue" phenomenon is treated with particular disdain.
You might find this hard to believe, but I can trace both fanfiction itself and the "Mary Sue" phenomenon to some of their earliest literary roots.
Fanfiction #1, in my opinion, is the Aeneid. Written by Virgil, a Roman, was heavily based on the works of Homer, a Greek (specifically, he wrote the Iliad and the Odyssey). It takes a minor element from the earlier stories (Aeneas) and expands on and explores it. Pure fanfiction.
"Mary Sue" #1 is Lancelot. A character unmentioned in the original works concerning the characters involved, Lancelot is a later addition to the mythos by a different author who plows his way past preexisting characters, has romantic liasons with the main character's significant other, and just generally wreaks havoc on the earlier works and preexisting dynamics.
Incidentally, I hate Lancelot.