Snapchat - Real-time picture chatting
Future Me - Send an email to your future self
Element.io - Secure messaging app with strong end-to-end encryption, advanced group chat privacy settings, secure video calls for teams, encrypted communication using Matrix open network. Riot.im is now Element.
Modern Memories - Letters written today. Delivered in the future.
StealthChat - StealthChat is Encrypted, Private, Secure Calls and Chats on Android and Apple iOS
Letter from You - Handwritten letter, mailed for you in the future.