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.
GroupSpaces - Free Group Website, Membership Database, Listserv Mailing List, Online Group Management Software : GroupSpaces
Mighty Networks - Mighty Networks enables entrepreneurs, organizations, and companies to create and grow a community-powered brand.
Bylde - Bylde is a SaaS platform you can use to Start, Manage and Grow your groups and communities.
Bigtent - BigTent is a web-based organization and communication portal for groups.