Bitmessage is a decentralized, encrypted, peer-to-peer, trustless communications protocol that can be used by one person to send encrypted messages to another person, or to multiple subscribers.
License: Open Source
Bitmessage is primarily focused on secure and private messaging without server storage, making it ideal for privacy-conscious users. Riot.im, on the other hand, offers a more complete communication platform with group chats and rich media sharing, making it suitable for team collaboration.