Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
Message Peeping Tom - Message Peeping Tom is an app by HSTAR, offers features that let you read your messages without giving the read report to the sender.
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
OpenKeychain - OpenKeychain helps you communicate more privately and securely.
Seecrypt - Seecrypt app allows users to do secure voice and conference calls by using advanced encryption features so they can contact their friends, colleagues, and family members without any privacy issues.