uLouder - Modern Craigslist and Nextdoor for urban nomads
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Explore nearby places for Airbnb - Discover must-see places when browsing Airbnb (Chrome ext)
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Whereabouts - Write posts and chat (group or 1-on-1) with people nearby!
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.