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.
Mighty Networks - Mighty Networks enables entrepreneurs, organizations, and companies to create and grow a community-powered brand.
Mobilizon - A free and federated tool to get our events off Facebook
Bigtent - BigTent is a web-based organization and communication portal for groups.
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Nabble - Nabble is a forum software that are embeddable and fully customizable with scripting language.