OpenKeychain - OpenKeychain helps you communicate more privately and securely.
Autocrypt - Convenient End-to-End Encryption for E-Mail
Tutanota - Get your encrypted mailbox for free.
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
GnuPG - GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP).