Eventjoy - The Easiest Way To Organize An Event
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Event Hunt - Startups and Tech Events Curated By Locals
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
TechEvents.co - Tech and startup events aggregator
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.