The Archive of Our Own aka AO3 is a multi-fandom archive built and run almost entirely by volunteers. It is currently designed to host text-based fanfiction. It is hosted on servers owned by the Organization for Transformative Works and therefore not vulnerable to a commercial hosting company deciding they don't like the fanworks.
Archive of Our Own gained popularity when FanFiction.Net stopped allowing Real Person Fics and started censoring Mature Content Fics. AO3 is very popular because it allows users to view fics as one single document instead of the separated chapters and even download stories as HTML, EPUB, MOBI and PDF files, making it great for people who want the stories on the go.
The archive was named one of Time magazine's 50 Best Websites of 2013.
South Park On AO3Edit
South Park can be found in the Cartoons & Comics & Graphic Novels category. As of August 12, 2020, South Park has over 600 stories. The earliest fanfiction is dated from 1998, but the no others were added until the website was launched in late 2009.