Cryptgeon - Securely share self-destructing notes
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
BURN.link - Send secure, encrypted, self-destructing messages
MyChat - Secure instant messenger for any network
Burner Note - Burner Note is a free, ad-free and open sourced service for sending secure text based notes that are completely erased from existence once they've been read.
Ubiq Security - Enable developers to build encryption into apps in minutes