My favorite and, in my opinion, the best site is Literotica.com. I spend a lot of time looking through the stories there. I've tried a few others but I didn't like any of them; the quality of the stories is very poor. My favorite categories are Masturbation & Toys (male masturbation stories are what I'm after) and I've found quite a few that were intelligent, sexy and believable and even a little witty.