Adamant Messenger - The most secure and anonymous messenger, encrypted with Blockchain
Dialog Messenger - handy and feature-rich enterprise multi-device messenger available for server or cloud – Slack-like, but not Slack-limited
Sonar - 2-way chat with your customers over SMS and FB Messenger
Ripcord - A desktop chat client for Discord and Slack
ttirik - Unsure about giving out your phone number, but still need to make a call?ttirik is a free service that allows you to make calls with just a URL link.
Ziggs - Smoothly Share Content Between Devices!