Herdict, an Internet censorship tracking service launched by Harvard’s Berkman Center for Internet & Society, has started aggregating their user-reported Internet blockages.
Iran and China stand out in the reported number of blockages of Facebook, Flickr, Twitter and YouTube. Interestingly, Mexico has quite a few blockages as well.
Check out the site for more detailed data and be sure to report when you are unable to access a site in your country!
Here’s an earlier posting I did with Jonathan Zittrain about the site.