Recs.
Updated
Element is an open source (Apache License 2.0) cross-platform instant messenger.
It is built on top of Matrix, an open APIs for decentralized communication, which has the objective of becoming a messaging standard, allowing different IM applications to communicate to each other.
SpecsUpdate
Pros
Pro Easy to use setup under Linux
Linux Clients available and own matrix-synapse Server possible under Linux.
Cons
Con Still in active yet early development
There is clearly a User Experience issue esp. in web version which the developers are aware of and are working hard to improve. They've got the concept concrete, so UX won't be an issue in no time. Most often used parts are functional, but some are yet not.
The success of riot depends on how fast the development is, or it'll get outdated too quickly as developers of Signal and Telegram have predicted.