Let's see now...
Back in 2001 before all that...unpleasantness that happened in NYC, I had posted my very first fanfiction on Fanfiction.net called "Heroes of Arcadia - The Gamemaster's Legacy". Back then, it was my first attempt at trying fanfic and for the source material, I had used something that was essentially the backstory for a paper and pencil RPG I was working on where you could roleplay a video game world. (Difficult to do in and of itself since this was before D20.) I was expecting the entire thing to flop since I was trying to write a SERIOUS crossover (crossovers were rarely ever serious back then) and had included a few other author's characters as well. I also included a self-insert character, which should have been an instant death knell.
But much to my surprise the fic became this sudden megahit. Dang near everyone left and right were lauding it despite the elements that should have killed it. Apparently, the fact that I focused more on the story rather than on the characters was a major plus, and several actually like my SI because apparently the older veteran gamer character was someone a lot of the readers could relate to.
Since then, the fic expanded into a full on series, with future fics that depicted life in the worlds of Arcadia. As a twist, however, several other authors started writing their own fics based on Arcadia...many of whom actually added to the main series itself, with the largest contributor being Ashura Hedgehog. Heck, I've even had the infamous Dr. Thinker write his own Arcadiafics!
Nowadays things have somewhat calmed down, since I haven't written anything new in five years, but I still get the occasional new person who put one of my fics on their favorites lists. More recently, however, a colleague of mine named FinalHazardJones actually included me on TVTropes' list of recommended Megacrossover Fanfiction. I am also presently working on a reboot of the Arcadia series which will include a more expansive multiverse right from the outset.
So...that's a measure of my greatest online achievement in a nutshell.