Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Calendify - Calendify is an event community where you can share, publish, and subscribe to event calendars. It helps you to stay informed about what you are interested in and what is yet to come.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Soulpicks - Discover places loved by friends and experts
Bigtent - BigTent is a web-based organization and communication portal for groups.
LinkedIn Events - A new way of connecting people from LinkedIn