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.
fangr.am - Building communities for creators
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
DogeHouse - Open-source audio chat on the web.
Bigtent - BigTent is a web-based organization and communication portal for groups.