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

TidyHQ - Organization management tool to support membership, subscriptions, event planning, and outreach.

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

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

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

Membee - Membee is web-based membership management software.