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

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

GroupSpaces - Free Group Website, Membership Database, Listserv Mailing List, Online Group Management Software : GroupSpaces

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

The Groups Place - The Groups Place is a new alternative to Meetup.

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