Wikipedia
GreatFire (GreatFire.org) is a non-profit organization that monitors the status of websites censored by the Great Firewall of China and helps Chinese Internet users circumvent the censorship and blockage of websites in China. The website also hosts a testing system that allowed visitors to test in real time the accessibility of a website from various locations within China. The organization's stated mission was to "bring transparency to the Great Firewall of China."
GreatFire has worked with BBC to make the Chinese-language BBC website available to users in China, despite it being blocked by the Great Firewall, by using a method known as collateral freedom that mirrored content on widely used content delivery networks, such as Amazon CloudFront and CloudFlare, so that it would be too economically costly for censors to block. The organization has since set up similar mirror sites for other blocked websites, such as Google and the New York Times, with a directory of links hosted on GitHub, which is currently not accessible in China.
GreatFire has been targeted with distributed denial-of-service attacks that attempt to take down the website by overloading its servers with traffic. In April 2015 it was targeted by a Chinese attack tool named Great Cannon that redirected massive amounts of Internet traffic to servers used by GreatFire.