Meetingbird - Powerful meeting scheduling and notes — right in your calendar

Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.

Standuply - Run daily standup meetings and track your metrics in Slack

Eventbrite - Discover Great Events or Create Your Own & Sell Tickets

Navigator - The Teamwork assistant — run great meetings without busywork

Bigtent - BigTent is a web-based organization and communication portal for groups.