Riot.im

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 https://alternativeto.net/software/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.