With a VPN it's possible to get around Internet blocks put in place by oppressive governments. Which countries are the worst offenders and why do they do it?

Citizens in free, democratic societies can often lapse into complacency, taking their civil liberties for granted. People living under oppressive governments, however, don't have such a luxury. The advent of the Internet means that it's easier than ever to stir up controversy, and some governments have tried to clamp down on the free expression and exchange of ideas on the Internet.
The offenders

Reporters without Borders, an organization dedicated to preserving the freedom of the press around the world, maintains a list of what it calls Internet enemies. The list is updated each year by Reporters without Borders. 2007's list is as follows, in alphabetical order:

* Belarus
* Burma
* China
* Cuba
* Egypt
* Iran
* North Korea
* Saudi Arabia
* Syria
* Tunisia
* Turkmenistan
* Uzbekistan
* Vietnam

China has long censored the Internet access of its 1.3 billion citizens. The nationwide block is informally known as the Great Firewall of China, and the system blocks sites of known dissident groups, international news sites, sites advocating the independence of Taiwan and sites linked to the Dalai Lama and the Tibet Independence Movement, along with the expected pornography and obscenity. A Harvard University study estaimates that some 18,000 sites are blocked by the Great Firewall. Among them, Wikipedia, Human Rights Watch and (no surprise here), Reporters without Borders themselves.

Other countries, like Syria, Egypt and Saudi Arabia, block sites containing content forbidden in Islam, specifically pornography, drug use and apostasy. However, these countries have also been known to block content based solely on political motivations.
The necessity of a VPN

For people living in the 13 countries on the Internet enemies list, using a VPN is essential to circumvent their government's unjust content blocks. By tunneling their Internet traffic through a VPN located in a free nation, surfers in these countries can browse the Internet as if they were sitting at a computer in the host country. A service such as Relakks (based in Sweden) or StrongVPN (based in the US) is a good choice for someone looking to break free from the constraints of censorship imposed upon them by their government.
Further reading

* Internet censorship (at Wikipedia)
* Internet Enemies List (at Reporters without Borders)
* Saudi censorship (Wikipedia)
* Check if a site is blocked in China