Bitmessage. The Most Anonymous Messenger.
Bitmessage is a free open-source messenger that has established itself as a reliable tool for conducting anonymous and secure communications online.
Myth
Any addressed message must have a recipient and a sender.
Reality
Bitmessage allows you to send messages without a recipient and a sender. Only the intended recipient can read it.
Why Bitmessage
The architecture of Bitmessage is arranged as follows: the message is sent to absolutely ALL computers in the network, but only the intended recipient can decrypt it. This makes it absolutely impossible to monitor user communications due to the lack of information about the recipient (and sender).
Recall the story of Caesar's cipher, when he sent his generals encrypted messages. Although the barbarians could not read the encrypted text, they could track with whom the correspondence was taking place and intercept the message intended for the recipient. Now imagine a situation where Caesar sends one message to all his generals and even to the barbarians; everyone receives this encrypted message, but only the owner of the decryption secret can read it. In this case, the recipient of the correspondence is unknown and cannot be established, and how would you know who has that secret?
Bitmessage uses public key encryption, so even the sender cannot decrypt the sent messages; only the recipient can. Encrypted messages are stored in the network for four days (with default settings).
Bitmessage is the only anonymous messenger. I am not aware of any other anonymous messengers. Many consider XMPP/Jabber to be anonymous, but this is not entirely true. XMPP/Jabber is not anonymous: there is a recipient, this recipient has an IP address that is logged by the server, there is data on when the recipient was online, from where they received messages, and one can infer how long the correspondence lasted. Bitmessage has no recipient, no server, and no logs.