Users in mainland China have had their Internet censored by the government for several years now, with more than 2600 websites being blocked at one time or another.

The reasons are as varied as the websites themselves, and China doesn't just block Internet websites. They monitor the Internet usage of their citizens. There are a number of people in jail - mostly journalists - on charges ranging from signing petitions to speaking out against the Chinese government.

According to greatfire.org, 44 of the Alexa Top 1000 websites are blocked in China. Here are the ten sites that are most popular with currently blocked users and the reasons why.

1. Various Google properties like Google Docs, Google Drive, Picasa, Gmail, and Google +: Most of these have been blocked since 2011 and are still blocked to this day. Picasa has been blocked since July 2009. This extraordinary censorship is because China appears to be trying to isolate the web available to Chinese users to only websites that China controls. However, this is bad news for web admins who miss out on valuable tools like Analytics.
2. Facebook: Facebook has been blocked in China for possibly longer than any other website. In July of 2008, the censoring started, and while users can still log onto Facebook using Virtual Private Networks and other tools, for most users, Facebook is a no-go. Most people agree that the reason for the block was the riots in Xinjiang four years ago. However, no further details have ever been released.
3. YouTube: The largest video-sharing website has been blocked in mainland China since March 2009. YouTube is blocked because it showed riots in Tibet that the Chinese government maintains were faked. Apparently, the Chinese government fears that some citizens may take their cues from a YouTube video and start a riot.
4. Twitter: Twitter has been blocked in China since 2009, and the reason is quite clear. It is easy for people to organize, share and communicate via Twitter, and there is very little the government can do to monitor or control it. This includes the Twitter domain t.co.
5. Wikipedia is unusual because it has been blocked and unblocked at least once throughout censorship history. The reason for the blocking is that users edit Wikipedia, and even though the Chinese version of events may be watered down or censored, users can change it to reflect actual events. All languages of Wikipedia are blocked, and Wikimedia images are as well.
6. Blogspot is also blocked in China, most likely because it is a Google property and a free and accessible platform for those who wish to speak out against the Chinese government to set up a blog to do so.
7. Xhamster is one of several pornographic sites blocked in China and one of the world's most popular free adult video sites. The reason for the censorship is that the government believes that removing pornographic materials will ensure Chinese citizens are "hardworking" and "moral." Xhampster is only one of several sites that are blocked.
8. Internet Movie Database (IMDB): While there has not been an official statement about why IMDB was blocked, many believe that a documentary called "When the Dragon Swallowed the Sun" is to blame. The film is about the Dalai Lama and the struggle to free Tibet.
9. NYtmes: While the reason for the IMDB block may be unclear, the reasons for the New York Times website censorship are easy to understand. The New York Times has been very vocal about the corruption in the Chinese government. It has posted multiple stories in both print and online publications, speaking out specifically against the money accumulated by the family members of Wen Jiabao, the nation's ex-Prime Minister.
10. RedTube.com: This is another pornographic website featuring adult material the Chinese government has blocked. As previously mentioned, the government feels that those looking at pornography will not be as morally upright and hardworking as those that do not view it.

These are the most popular ones. If you, however, want to check whether a particular website is visible behind the Great Firewall of China, you can do so with our free tool.

Pro Tip: Check out how to access blocked websites.