The OpenNet Initiative says that 30 or more countries are actively engaging in internet censorship now.

According to ONI,

"China has earned the dubious distinction as the world leader in filtering Internet content. The Chinese state-run Internet censoring system is without parallel, both in the technical sophistication of the filtering apparatus and in the breadth of topics subject to blocking."

This PBS posting from last April describes how filtering works in China:

Government filtering occurs at the nine Internet Access Points (IAPs) where China connects to the global Internet. These IAPs provide bandwith to hundreds of Internet Service Providers (ISPs), which must register with the state and censor the Web sites on their services. "It's not a single organization building these lists [of blocked addresses, domains and keywords]," says [Nart] Villenueve [Univ. Toronto]. "Each ISP is required to block bad Web sites, but they decide themselves how to implement this requirement."

Cyber cafes are also required to register and monitor users, and citizens with home Internet access must register with local authorities.

Here's a recent story about a Canadian's experience with the Great Firewall.

China is tightening controls on internet news content.  According to ONI:

The three most significant changes implemented in [a recent law] are adding categories of prohibited news Web site content (inciting illegal assemblies or conducting activities on behalf of illegal civil organizations), banning non-government opinion and analysis pieces, and greatly increasing requirements for individuals and small groups posting news.

The two new categories of banned material aim to discourage use of the Internet for political organization and mobilization, which are viewed by the Chinese state as subversive. Thus, for example, the increasing use of mobile phone messaging to organize protests not only violates these new regulations, but also falls under several established categories of prohibited content: harming the interests of the nation, disrupting the solidarity of peoples, and disrupting national policies, at least. Rather than changing the legality of using the Internet for "subversive" organization, these new regulations fortify state control over expression on the Internet and serve as a powerful reminder and warning against using the Internet for purposes the state views as threatening.

Banning non-government news commentary primarily affects major search engines and portals that are licensed to publish governmental news and certain types of approved, independently-gathered news. These sites must now stop posting commentary and analysis --except pieces generated by state news agencies -- effectively limiting Internet news to government-created or sanctioned news.

The blocking of internet sites is a human rights issue, and ONI's information is crucial.