Smashwords dot com - has PDFs, text, mobi and epub files of lots of stories of all topics (adult and not) and many are free. Others make a little something for the authors - Robert Lubrican was mentioned earlier; I've been reading his stuff for literally decades now and like to throw him some cash now and then for the great pleasure he's given me, no homo!
Also, I heartily recommend Ann Douglas - hot lesbian & straight erotica since the 1990s...