Element / Matrix
Element (previously Riot.im) is a libre instant messaging client based on the Matrix protocol. It includes text chats, audio/video calls and file transfers.
The Matrix protocol has a federated design and allows bridges to other communication apps. Furthermore, anyone can set up their own Matrix server and use it to collaborate with other people’s Matrix servers.
Mumble is an open-source low latency, high quality voice chat application. It also has text chats and is popular amongst gamers and podcasters.