For instance: late last year, I suddenly found that I couldn't reach the New York Times web site from my apartment in Beijing without using a VPN, and I heard from a friend in Shanghai that she was having trouble too. We didn't know if it was a problem on the Times's end, coincidental problems with our local connections, some other unknown issue -- or a conscious crackdown in China. As it emerged, Chinese officials had imposed a nationwide blackout on the NYT site. But it took a while to determine what was going on.
Herdict is meant to be a quick, crowdsourced way of reporting such developments -- and it has recently come out with Chinese and Arabic language versions of its site. It looks as if it's getting more traffic than the last time I checked a few months ago, but it could use more participants to produce finer-grained reports. Even now it's a quick way for people in, say, China to figure out that if they're having problems reaching YouTube, Blogger, China Digital Times, or Huffington Post, the fault lies not with them but with the Great Firewall. A useful tool.