This tag refers to the thorny topic of certifying software testers, rather than public key certificates.
There are a variety of companies who offer tester certifications. There are a variety of criticisms of every one of them. This is a hotly debated topic in the testing industry. It's worth restating StackExchange is not a debating site - it's not the place to have an extended argument. Please bear this in mind for any questions you raise or answer under this tag.
This overview by Cem Kaner on credentialling in software testing offers a cogent summary of the current state of affairs, and his opinion on how things will develop. It is worth reading whatever your position on tester certification.