qTox - A Qt graphical user interface for Tox.
Mumble - Mumble is an open source, low-latency, high quality voice chat software primarily intended for use...
Tox - Send messages, files, create groups and make Tox to Tox audio and video calls. Securely.
Discord - Step up your game with a modern voice & text chat app. Crystal clear voice, multiple server and channel support, mobile apps, and more.
Toxic - Toxic is a client with an ncurses interface, written entirely in C.
RaidCall - Free & Light. Setting up is fast and easy; with no need to rent or set up servers. Low CPU usage and minimum memory footprint allows for smooth, uninterrupted communication. im. Group Communication.