Franz -
Franz is a free messaging app / former Emperor of Austria and combines chat & messaging services into one application. Franz currently supports Slack, WhatsApp, WeChat, HipChat and many more. You can download Franz for Mac, Windows & Linux.
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Singlebox -
Singlebox for Mac and PC is an all-in-one messaging & emailing app - a single place for all your web services and accounts, including Discord, WhatsApp, Messenger, Slack, Telegram and many many more.