Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Chadder - Encrypted messaging app
Messenger Platform - Discovery, chat extensions, and richer experiences
GrapheneOS - GrapheneOS is an open source privacy and security focused mobile OS with Android app compatibility.
Expensive Chat - Chatroom where you pay per letter
Earlynote - A time-travelling messaging app