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.
GroupSpaces - Free Group Website, Membership Database, Listserv Mailing List, Online Group Management Software : GroupSpaces
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
The Groups Place - The Groups Place is a new alternative to Meetup.
Cvent - Cvent's event management software provides event planners with a complete solution to increase event attendance and decrease event costs.