Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.

Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.

Kommunity - Explore communities that share your passion with millions of people

DownToMeet - DownToMeet is a platform to organize, find, and attend group events.

Mighty Networks - Mighty Networks enables entrepreneurs, organizations, and companies to create and grow a community-powered brand.

Get Together - Get Together is an open source event manager for local communities.