Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
TechEvents.co - Tech and startup events aggregator
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Facebook Local - Discover places to go and events happening near you
Bigtent - BigTent is a web-based organization and communication portal for groups.