Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Mobilizon - A free and federated tool to get our events off Facebook
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Get Together - Get Together is an open source event manager for local communities.
Bigtent - BigTent is a web-based organization and communication portal for groups.