I have been using this website for the past ten years. Though lately my use has tapered off significantly, as I rarely write fanfiction anymore, and what I do write tends to make its way onto Livejournal instead, this website has been absolutely invaluable for me for the past decade. It is a huge database of fanfiction--for those who don't know fanfiction is derivative fiction, essentially fiction using characters that the writer did not invent. Though there are some moral objections to the creation of fanfiction, I believe it's a great way to hone one's writing skills, as well as to prolong one's interest in a novel, or television show, or other media, and to understand it more deeply and discuss it with other fans.
Fanfiction.net hosts some spectacular work in this area. It has its share of bad writing, of course. There is no quality control, anyone can post anything they want, though pieces will sometimes be deleted if they don't seem to treat the rules of spelling and grammar with any respect. It can be hard to sift through the rough to find the diamonds, but when you do it is well worth it.
This website is easy to use--I was using it at age eleven with no trouble. It's easy to upload your own work, and it's easy to find what you're looking for, with their extensive search function and their well-organized categories. There is also a beta service, where users can declare themselves willing to edit other user's writing, and those looking for editors can ask them accordingly--I have beta'd for others and found betas for myself in this way. Users also have the ability to create their own forums, on any topic they choose. Fanfiction.net also has a sister site, fictionpress.com, where users can post their original work.
Overall I'd say Fanfiction.net is a wonderful website. Amongst fandom it is often reviled as "the Pit" or "the pit of voles" because of the often poor quality of the writing hosted on it. This is not a reputation that I feel is deserved--there is beautiful writing there if you take the time to look for it.
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