It really depends on your idea of what "fanfiction" is. By the traditional definition, I have written fanfiction, but only because I've used a universe from something already established. I think I've really only written one fanfic, a Pokémon one, however it didn't involve any of the anime characters – only the game events. And the only character from the game who appeared in it was Red. Everyone else was either original or nameless (i.e, nameless Rocket Grunts).
I have read fanfiction, though, mainly novelizations. I remember there was actually a decent novelization of Golden Sun on fanfiction.net. I have no idea if it's finished or has been abadoned, since it's been a long time since I've been there. In fact, I just recently found the email I got for joining it back in 2005.