Contrary to Wintersweet's writeup on Undocumented Features, there does exist good anime-themed fanfiction on the internet. I would argue that in addition to the Eyrie's excellent literary (if you can call it that) ventures, authors like Richard Lawson with literate and thought-provoking works such as Thy Inward Love, Thy Outward Part, and Transitions do a remarkable job of extending their respective stories in immensely creative ways.

Which is not to say that the vast majority of fanfic out there isn't pure crap. Don't believe me? Read rec.arts.anime.creative, and don't come crying to me when your head explodes.