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
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Howbout - Social calendar app to easily make plans & find availability
Bigtent - BigTent is a web-based organization and communication portal for groups.