OpenSSL

remove passphrase from private key
openssl rsa -in asdf.key -out asdf.key
show technical infos about private key
openssl rsa -in asdf.key -text
show certificate details
openssl x509 -in asdf.cert -text
show certificate of a mail server (STARTTLS)
printf "QUIT" | openssl s_client -connect mail.example.org:25 -starttls smtp | openssl x509 -text -noout
show remote certificate dates
printf "\r\n" | openssl s_client -connect example.org:443 2>/dev/null | openssl x509 -dates -noout
generate CSR (length: 4096bit)
openssl req -newkey rsa:4096 -keyout asdf.key -out asdf.csr
create pkcs12 file from cert an private key

(ex. for Thunderbird S/MIME with StartSSL Cert) 

openssl pkcs12 -export -out asdf@example.org.pfx -inkey asdf@example.org.key -in 2_asdf@example.org.crt -certfile 1_Intermediate.crt