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

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

Galette - Galette is a membership management web application towards non profit organizations.

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

Bigtent - BigTent is a web-based organization and communication portal for groups.

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