|·GlobalVoices ADVOCACY has a page they call the Access Denied Map. On it they track visually, including pop-up annotations, countries that prohibit access to web sites.
The thing that made the biggest impression on me is the number of countries that block bloggers or Flickr. (You can check this yourself by going to their site and clicking the pushpins on their Google map.)
|·Opennet.net also tracks blocking/filtering worldwide. They look at the reasons given for filtering and compile maps tracking four different types of filtering.
|Reference: UN International Covenant on Civil and Political Rights (ICCPR) and commentary