One of the most versatile SSL tools is OpenSSL which is an implementation of the SSL protocol.
This app is an OpenSSL client for the web browser. It only works locally without going through the server. You can convert certificates or run OpenSSL commands instantly in your browser.
This app supports multiple features including Print Certificates, Convert Certificates, Checking Using OpenSSL, General OpenSSL Commands, and more.

It also has hundreds of different functions that allow you to view the details of a CSR or certificate, compare an MD5 hash of the certificate and private key, verify that a certificate is installed properly on any website, and convert the certificate to a different format. 
Supported formats: cer, crt, der, p7b, p7s, p12, pem, pfx, csr, key...