Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Guestboard - Lead powerfully-organized group events with Guestboard – a modular event platform that gives groups of 10-1000+ people a central space to coordinate, build excitement, and save money. No more crazy email threads. No more massive group texts.
TidyHQ - Organization management tool to support membership, subscriptions, event planning, and outreach.
Mobilizon - A free and federated tool to get our events off Facebook
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.