uTox is lightweight Tox (https://github.com/irungentoo/ProjectTox-Core)
client written in C.

Simple and convenient program for confidential communication.

Still early in development, though it already supports some of the most
important features, like file transfers, A/V, group chats, and DNS Discovery.

There are also binaries for other OS: Linux, Android, OS X and Windows, which
makes it possible to communicate between users of different OS.