My fanfics:

Mouthwashing

Other original texts that are not fanfiction:

Chronicle

Star Trek non original work special section, an homage to the start of modern fanfiction:

A Fragment Out of Time by Diare Marchant (1974) [NSFW]

First slash fan fiction, since slash ships started with the ST fandom, with Kirk/Spock (or K/S, also known as "The Premise") specifically. Published in the zine "Grup" #3.

A TREKKIE'S TALE by Paula Smith (1974)

"Mary Sue" archetype's name origin.

The first Star Trek's fan stories, printed in fan zines specifically, were the ones from Spockanalia's issue #1, from 1967.

World's first known "fan fiction" writer, Publius Vergilius Maro (Virgil). His "Aeneid" is an Homer's "Odyssey" fanfic. Hey, I also shout-out to Dante Alighieri for "The Divine Comedy"!