Here's a page listing status of the invisible systems used by governments and ISPs around the world to "block child pornography." The lists of "child porn" are generally secretly compiled and held, and the blocks often simply return a timeout or 404 error, so it's virtually impossible to determine whether a given page is being filtered by the system. The potential for abuse is incredible, since pages that get added to the list effectively disappear and there's practically no way to appeal the censors' decision (what's more, if you do appeal and lose, you're branded a pedophile).
"Child porn" lists have historically included literature from Nabokov's "Lolita" to Alan Moore's brilliant "Lost Girls". These lists also often pick up material made by gay/lesbian/bi teenagers: journal entries and videos and audio describing their experiences with authority and mainstream society. These kids are even more poorly equipped than Nabokov fans to advocate for their right to communicate with their peers over the Internet.
(Thanks, SethF!)
