Wire (https://wire.com/) is end-to-end encrypted, based on Signal protocol. It's open-source and cross-platform with mobile apps. I've been using it instead of Skype for two years and I don't regret that decision.
Moxie Marlinspike has commented that Wire does not use the Signal Protocol but only took some components from it and created their own protocol, which he did "not recommend" https://news.ycombinator.com/item?id=12690148 Has there been any changes to that situation?