GnuPG - GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP).
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
PGP Tool - PGPTool allows you to encrypt and decrypt files. It's free and easy to use.
Skizzle Email - Skizzle is the most secure way to share end-to-end encrypted files through email, backed by blockchain