Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Natural Atlas - An evolving, nationwide map of the outdoors
Facebook Local - Discover places to go and events happening near you
Bigbee - Connect with anyone, anywhere.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
The Nudge - A planner friend in your pocket.