Update (2018-05-08): The GreatFirewallofChina.org project has been acquired by Comparitech and the tool has moved to https://www.comparitech.com/privacy-security-tools/blockedinchina/
Someone has set a server somewhere in China to receive a URL and then act as a web client to check whether or not this URL is accessible. The implication is that some URLs are being blocked by some process in some parts of China. Maybe. It's difficult to distinguish between technical problems and political interventions from here (to be fair, the creators of this site to make a disclaimer to this effect) The site has a funky animation showing the HTTP request/response crossing a map of the world - reminiscent of so many movies where the 'hacker' does something clever. The URL to the home page of UKOLN where I work is unavailable to that server in China. I also checked this blog's address and it is currently available, but perhaps it will be blocked now that I have blogged about this! I must remember to check in a few days.... To check whether or not you are being censored somewhere in the world, go to www.greatfirewallofchina.org and enter your URL.