aTox is a free open-source messaging app for the Tox protocol.

Why aTox?

Open-source: free to share, inspect, and change as you like.
Reasonable: sane defaults, no guesses about what you care about, doesn't ignore what you want.
End-to-end encrypted: the only people who can see your conversations are the people you're talking to.
Distributed: no central servers that can be taken down or compelled to turn over data.
Lightweight: no telemetry, ads, or other forms of monitoring.

More info: aTox, Tox.