Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
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.
YAATA - YAATA app offers features that let you send multimedia files such as images, videos, audio, and GIFs, etc.
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.