For me, it was LOTR. I was following a story on Of Elves And Men, and emailed the author, since it was incomplete. She gave me a link to her LJ, and I joined in April of 05. I found some HP friends that ended up there because of the Silver Snitch implosion, and came over here when LJ went all stoopid.
I still adore my Lucius. I named my laptop after him.