Wire, an alternative messaging app to WhatsApp and Skype is now fully open source. The server-side code has been opened in April and today it is now the client-side to be opened.
The app is working on all major desktop and mobile platforms.
Erratum: I misread the article's date: Wire's client has been open sourced on July 22th 2016 and not today (July 22th 2017). I did a little mistake. :D