If you haven't already stumbled upon this amazing site, you are missing out!
Although there is the far better known FanFiction.net which is an incredibly well-visited and often added to archive of amateur authors and varied readers, it has recently updated its submission guidelines to exclude any Mature or Adult-rated material. Sad, I know. I used to visit this site often to read adult fan fiction for both Pushing Daisies and Firefly, long gone shows, but still remembered fondly. I can still find a few of the old stories that have been overlooked, but no new submissions. When I was searching out a place for my own works-in-progress - a place to pick up readers and solicit comments on my writings, I first started here.
Then, I found out quickly I was not welcome and had to go elsewhere. So, through trial and error, a couple nights of Internet searching, etc, I stumbled upon Adult Fan-Fiction. This site is not as clean, well-organized, or even accessible as the other site, but it does allow, in fact encourages, Adult and Mature subjects. So, I settled here.
And, I explored. I found good stuff - of course, there is some real nasty, badly written, unedited, and plain disturbing stuff, too - and have a few authors I already follow near religiously. Though the site, itself, is lacking, the writers inside are gold, pure gold. You can find just about anything, mainly fiction composed on other well-loved literary works, but that would be selling the site short. Lots of unique stories exist here, as well. And, those are the ones I love the best!
Take the time, some evening, when the snow is deep and the winds are howling, and settle down to a sensual search through the archives. You won't be disappointed.