Certificate Transparency is a project started by Google to provide complete transparency in the process of issuing SSL certificates, in order to avoid abuse or prevent damage from a compromised Certificate Authority.
This goal is achieved by creating a publicly verifiable and available log of all certificates signed by a Certificate Authority. This allows anyone to get a complete list of all the certificates and verify that they are indeed valid.
Comodo created a search engine to allow quick access of this list by anyone. It can search for domains, subdomains, and it also supports wildcards.