Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Tettra - Tettra is a company wiki that helps teams manage and share organizational knowledge.
Facebook Local - Discover places to go and events happening near you
Slinky - Your slack links — organized!
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Obie.ai - Access knowledge quicker without leaving Slack