Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
uLouder - Modern Craigslist and Nextdoor for urban nomads
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Explore nearby places for Airbnb - Discover must-see places when browsing Airbnb (Chrome ext)
Bigtent - BigTent is a web-based organization and communication portal for groups.
Whereabouts - Write posts and chat (group or 1-on-1) with people nearby!