FanFiction.net, sometimes negatively referred to as the 'Pit of Voles', is without a doubt the largest fanfiction site on the internet. It boasts thousands of stories across hundreds of fandoms, and is home to some of the greatest fanfiction writers in any of them. The Harry Potter section alone has over 300,000 fics.
Most writers currently hosted at Unknowable Room once posted their stories at FanFiction.Net. The website's unclear and often arbitrary deletion policies caused the first of them (Sarinileni) to leave it, and many followed. (see History of Unknowable Room for more details)