Nakama - Nakama is an open-source distributed social and realtime server for games and apps.
Game Center - Game Center brings social gaming to OS X and iOS apps.
Discord - Step up your game with a modern voice & text chat app. Crystal clear voice, multiple server and channel support, mobile apps, and more.
TeamTalk - TeamTalk 4 is a conferencing system which enables a group of people to collaborate and share...
GSC - With GSC, you can pick from any of 14 worldwide datacenters to host your own free voice server.
Twitch Messenger - Desktop and mobile app for Twitch users to communicate with friends and teammates across platforms.