CozyCal - Online appointment scheduling
Meekan - Connecting the world's calendars
Sorted - Sorted³ has been crafted to make scheduling quick and easy. Try it for a few days and it may very well become a productive, yet enjoyable part of your day.
RobinPowered - Robin coordinates meeting spaces, people, and things in your office. Complete with analytics and insights that uncover usage and help optimize space.
Kyber for Slack - All-in-one Slack app: tasks, standups, polls, surveys & more
FreeBusy Scheduling Assistant - Coordinate meetings 10x faster from Skype, Slack, Echo