Watch over SSL issuance on your domain

ct_advisor is a proactive alerting tool for Google's Certificate Transparency

Certificate Transparency is a service that provides an audit trail of all issues SSL certificates. It compliments the use of strong SSL configuration and various security headers, it assists in identifying certificates that were not authorised for your domain. In general, CT is utilised in a reactive fashion for investigating incidents. However, this service continually polls the CT log, and will proactively trigger alerts and send you an email if a certificate is ever registered for your domain, by any CA in the CT program.