This article in Foreign Policy lists five democratic countries that censor web content--and South Korea is among them.
As described in the article, the South Korean web censorship mechanisms are mostly directed at blocking any and all propaganda from the North, though ISPs carry out a lot of protect-the-children censorship. Of course, like most censorship programs, South Korea's casts a much wider net than necessary--CareerBuilder.com, for example, seems to be blocked in the ROK.
Given such strange cases, I can't help but wonder how many "accidentally" blocked or crippled sites simply have powerful domestic competitors here? I'm looking at you, Naver! Fastest web portal in Seoul!