Mighty Networks - Mighty Networks enables entrepreneurs, organizations, and companies to create and grow a community-powered brand.
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Bigtent - BigTent is a web-based organization and communication portal for groups.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Nabble - Nabble is a forum software that are embeddable and fully customizable with scripting language.
Calendify - Calendify is an event community where you can share, publish, and subscribe to event calendars. It helps you to stay informed about what you are interested in and what is yet to come.