I just gave this a quick run. There aren't many options; however, it's very easy and quick to use. It can definitely suffice many use cases.
Doodle - Make meetings happen. With Doodle, scheduling becomes quick and easy.
Dialog Messenger - handy and feature-rich enterprise multi-device messenger available for server or cloud – Slack-like, but not Slack-limited
WhenIsGood - Plan the time to have your meeting or event by co-ordinating availability with all the particpants using this fuss-free online tool.
Ripcord - A desktop chat client for Discord and Slack
Framadate - Make your polls
Done Hui - No need to switch between multiple pieces of software to get through the workday. CHATS: Communicate freely. CALENDAR: Know your team's availability, plan meetings. No more conflicts. TO-DOs: Stay on top of all projects. FILES: All files, one spot.