Features vary between clients but may include voice and video chat, file sharing, and even desktop sharing.
Tox is a peer-to-peer instant-messaging and video-calling protocol that offers end-to-end encryption.
Tox Protocol (Wikipedi) - https://en.wikipedia.org/wiki/Tox_%28protocol%29
The idea was to create an instant messaging application that ran without requiring the use of central servers. The system would be distributed, peer-to-peer, and end-to-end encrypted, with no way to disable any of the encryption features; at the same time, the application would be easily usable by the layperson with no practical knowledge of cryptography or distributed systems.
About - Tox - https://tox.chat/about.html
Tox relies on encrypted peer-to-peer networking to chat with others, eliminating the need for messages to travel through a central server. Skype itself used to be based on p2p networking, but since Microsoft acquired the company, Skype has been moving away from p2p towards centralized servers.
Meet Tox, an open, privacy-focused Skype replacement | PCWorld - https://www.pcworld.com/article/2600419 ... ement.html
Clients in the database
Miranda NG (with plugin)
Not in the database
Privacy-enhanced Instant Messaging