tripcake - Doodle for travel plans with friends
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
DUNK - Surprise your friends with secret group plans
Facebook Local - Discover places to go and events happening near you
Howbout - Social calendar app to easily make plans & find availability
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets