4chan. I used to browse the hentai section of 4chan rather frequently, and on several occasions I'd run into a h-manga that I liked, downloaded it and it would be labeled [FAKKU]. I finally got curious and typed Fakku into Google and lo and behold. I'd be lurking for several months before one of my friends who is on this site
(ponmaster) told me I should join. I didn't realize that most of the content was locked away for members, so I decided to join.