Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
Easy Message - Easy Message app comes up with features to help users in effortlessly sending text messages to a phone number, which is not in the contact list.
OpenKeychain - OpenKeychain helps you communicate more privately and securely.
Cellcrypt - Cellcrypt app provides features so you can enjoy secure conference calling, messaging, and sharing of documents.