Mumble!

All-in-one voice and text chat for gamers that’s free, secure, and works on both your desktop and phone. Stop paying for TeamSpeak servers and hassling with Skype.

Riot (formerly Vector) is an opensource interoperable app running on Web, iOS and Android. It is built around chat rooms, both public and private and provides messaging, filesharing, voice and video conferencing (beta on Android and web), integrations to tools like GitHub, Jira, Jenkins and much more soon. Vector is built on the Matrix.org open standard which provides bridges to other systems such as Slack, IRC and a lot more soon. One can use the hosted service or run their own. »

TeamSpeak is a proprietary Voice over IP software that allows users to speak on a chat channel with other users, much like a telephone conference call. A TeamSpeak user will often wear a headset with an integrated microphone. Users use the TeamSpeak client software to connect to a TeamSpeak server of their choice, from there they can join chat channels.The target audience for TeamSpeak is gamers, who can use the software to communicate with other players on the same team of a multiplayer game. Communicating by voice gives a competitive advantage by allowing players to keep their hands on the controls. »

Formerly known as Curse, Twitch Messenger (also Twitch Desktop App) was acquired by Twitch and rebranded as the desktop and mobile app for Twitch-based messaging. It's a fast, secure and free communications app for and by gamers.

Technitium Bit Chat is a secure, peer-to-peer (p2p), open source instant messenger designed to provide end-to-end encryption. Primary aim of developing this instant messenger is to provide privacy which is achieved using strong cryptography. It can be used over Internet and private LAN networks for instant messaging and file transfer.

dialog app is built and designed for corporate communication on all types of mobile devices - tablet, mobile phones etc.

Ventrilo is a real-time voice chat software for group communications with surround sound support.

Cyanide is Tox client for Sailfish OS .

GameVox provides group voice communications for gaming communities.

Small program mainly aimed to provide a simple user interface for making VoIP calls with Asterisk PBX (an open source VoIP PBX).

Zello (formerly loudtalks) the fastest way to communicate online. Don't type or call — just push F7 and speak! Reply, when convenient, talk to all people in a group at once or anyone individually, re-play the entire conversation later from voice history. It works on Blackberry, iPhone, Android and Windows Mobile devices too.

uMurmur is a minimalistic Mumble server primarily targeted to run on embedded computers, like routers, with an open OS like e.g. OpenWRT. The server part of Mumble is called Murmur, hence the name uMurmur. It is available as a precompiled package for quite a lot distributions. Check your distribution's package repository.