Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Mighty Networks - Mighty Networks enables entrepreneurs, organizations, and companies to create and grow a community-powered brand.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Bigtent - BigTent is a web-based organization and communication portal for groups.
Meetabit - All the local tech and startup meetups in one place - easy to organize & attend
Nabble - Nabble is a forum software that are embeddable and fully customizable with scripting language.