Dialog Messenger - handy and feature-rich enterprise multi-device messenger available for server or cloud – Slack-like, but not Slack-limited
FreeBu - Smart free/busy calendar sharing and meeting organizer.
Ripcord - A desktop chat client for Discord and Slack
Letsfind - Letsfind is a fast, easy-to-use, ad-free way to create online (meeting) polls.
Ziggs - Smoothly Share Content Between Devices!
FullCalendar - FullCalendar is an open-source JavaScript event calendar.