Eventbot - A group calendar made for Slack
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Clockwise for Slack - Tame the chaos of work
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Teamweek - A project management software to help you get the best overview. Plan fast, change plans faster!
Bigtent - BigTent is a web-based organization and communication portal for groups.