Dialog Messenger - handy and feature-rich enterprise multi-device messenger available for server or cloud – Slack-like, but not Slack-limited
Doodle - Make meetings happen. With Doodle, scheduling becomes quick and easy.
Ripcord - A desktop chat client for Discord and Slack
Framadate - Make your polls
Ziggs - Smoothly Share Content Between Devices!
Dudle - Dudle is an online scheduling application, which is free and open source.