EventsBase - Find the best professional events nearby or globally
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Rally by Eventbrite - Find the best local events and rally your friends to join
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Calendall - Follow the best events online, without leaving your couch.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets