With the rise of widespread government monitoring programs, Tox is an easy to use application that allows you to connect with friends and loved ones without anyone else listening in.
Antox is an Android client for Tox. It aims to bring the full multimedia support Tox offers to your device, although it's still currently in heavy development.
µTox is the lightweight client with minimal dependencies; it not only looks pretty, it runs fast! µTox is available for Windows, OS X, and Linux (Android support is experimental), with full support for chat, file transfers, audio/video calling, and desktop sharing...
The project seems no longer developed. Last version, released in January 2017, can be still downloaded from GitHub
Secure and private open source peer-to-peer Instant Messenger.