Huntathon - Browse and submit Product Hunt Global Hackathon projects
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
CoFoundersLab - Meet 250,000+ entrepreneurs online
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Hackathon Match - Find collaborators for the Product Hunt Global Hackathon
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.