Mumble - Mumble is an open source, low-latency, high quality voice chat software primarily intended for use...
Pidgin - Pidgin is an easy to use and free chat client used by millions. Connect to AIM, MSN, Yahoo, and more chat networks all at once.
Discord - Step up your game with a modern voice & text chat app. Crystal clear voice, multiple server and channel support, mobile apps, and more.
Trillian - Trillian is a decentralized and federated instant messaging platform that lets your whole company send private and group messages, keep tabs on what co-workers are doing, share files, and much more.
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.
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.