So-called "anonymous" open proxies allow a user to conceal their his or her IP address from the accessed service. This is sometimes used to abuse or interrupt the accessed service, potentially violating its terms of service or some laws; open proxies are therefore often seen as a problem. However, anonymous open proxies increase the user's anonymity or security when browsing the web or using other Internet services: a user's true IP address can be used to help deduce information about that user as part of an attempted computer crack. Furthermore, open proxies can be used to circumvent efforts at Internet censorship by governments or organizations. Several web sites provide constantly updated lists of open proxies.

Internet censorship is control or suppression of the publishing or accessing of information on the Internet. The legal issues are similar to offline censorship. One difference is that national borders are more permeable online: residents of a country that bans certain information can find it on websites hosted outside the country. Conversely, attempts by one government to prevent its citizens from seeing certain material can have the effect of restricting foreigners, because the government may take action against Internet sites anywhere in the world, if they host objectionable material.

Barring total control on Internet-connected computers, such as in North Korea, total censorship of information on the Internet is very difficult (or impossible) to achieve due to the underlying distributed technology of the Internet. Pseudonymity and data havens (such as Freenet) allow unconditional free speech, as the technology guarantees that material cannot be removed and the author of any information is impossible to link to a physical identity or organization

http://www.efa.org.au/Issues/Censor/cens3.html
http://www.religioustolerance.org/cyberpat.htm
http://en.wikipedia.org/wiki/Internet_censorship

Divided into different countries.
Description of different blocking softwares in use.
More reading at wikipedia. Extensive...