Mumble - Mumble is an open source, low-latency, high quality voice chat software primarily intended for use...
Empathy - Apps/Empathy - GNOME Wiki!
Discord - Step up your game with a modern voice & text chat app. Crystal clear voice, multiple server and channel support, mobile apps, and more.
IM+ - Chat everywhere! IM+ is the most widely used mobile IM app! Connect all-in-one to MSN, Yahoo!, Goo...
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.
Gajim - Full featured and easy to use Jabber client