Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Planmesh - The easiest way to plan with friends
Event Hunt - Startups and Tech Events Curated By Locals
tripcake - Doodle for travel plans with friends
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Onin - Plan events without leaving the conversation