FanFiction.Net is the self-proclaimed "world's largest fanfiction archive and forum" and is one of if not the largest site hosting fanfiction. Has a sister site Fictionpress, much smaller, identical layout.
It is in the process of being preemptively archived.
Most of the work is just staying on top of the new stories and user profiles.
Profile IDs are being scraped from indices; downloading will then occur on a per-profile basis. Drop by the IRC channel EFNet #fanfriction for more information.