Lots of authoritarian regimes impose restrictions on the types of information their citizens can access or publish on the internet. Some countries systematically limit the available websites, and others only do so when their citizens use the internet to organize protest actions (as was recently the case in Iran, Tunisia and Egypt).
China is often criticized for its large-scale and systematic filtering (dubbed the Great Firewall of China), but the phenomenon is relatively widespread. Here are some maps showing the extent of internet censorship:
internet filtering of social content
internet filtering of security content
internet filtering of political content
(source, where you can also find more detailed information)
And this is the index of Reporters Without Borders:
reporters without borders map of internet censorship
And the 2011 version:
map of internet censorship
More data on this are here and here. More on free speech and the internet is here. Something about the related topic of internet access rights is here. More human rights maps in general are here.