GnuPG - GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP).
Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
YAATA - YAATA app offers features that let you send multimedia files such as images, videos, audio, and GIFs, etc.
PGP Tool - PGPTool allows you to encrypt and decrypt files. It's free and easy to use.
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
Autocrypt - Convenient End-to-End Encryption for E-Mail