FanFiction.Net
http://www.fanfiction.net
http://www.fanfiction.net/~tatianamik
It’s a guilty pleasure. Sometimes you just don’t get enough of the characters you love on TV or in the movies. So that’s why folks started writing fanfiction. This is a repository of FanFiction from various authors of varying quality.
FictionPress
http://www.fictionpress.com
http://www.fictionpress.com/~tatianamik
Sometimes you want just a good old fiction story, but without the book. This is styled similar to FanFiction, but is for original fiction.
Ficlets
http://www.ficlets.com
http://ficlets.com/authors/tatianamik
Ficlets are short, short stories. The maximum it can be is 1024 characters. Not words, characters. The minimum is 64. But you can add prequels or sequels to it. And others can write prequels/sequels to it as well. A good example of this is A Godawful Small Affair by Wil Wheaton that people have added prequels and sequels to. Yes, that Wil Wheaton.