Canary Watch was created by the Electronic Frontier Foundation (EFF), NYU's Technology Law & Policy Clinic and other privacy advocate groups. So far, it's only tracking a handful of sites, including secure cloud platforms like Rsync.net and Spider Oak, along with social/news sites Tumblr, Reddit and Pinterest. The EFF points out that most folks don't pay any attention to the transparency "canaries" (or even know they exist), so Canary Watch tracks any changes or disappearances and explains what they mean.
Canaries allow service providers to be as honest and transparent with their customers as they are allowed to be by law.
For instance, Reddit says that as of January 29th, 2015, it has "never received a National Security Letter, an order under the Foreign Intelligence Surveillance Act, or any other classified request for user information." Were that statement to disappear, Canary Watch would flag it, indicating that the site was likely served a warrant. There would be no way of knowing what the request was, however, unless Reddit successfully fought to unveil it (which it says it will do).
For people that follow privacy issues closely, the new site doesn't bring anything new. However, as the EFF told Threatpost, canaries "allow service providers to be as honest and transparent with their customers as they are allowed to be by law." More importantly, it brings the concept of canaries to the public and may help motivate sites to better protect your privacy from government overreach.