Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
NationBuilder - The world's first platform for leaders, handcrafted from scratch to help you grow your community and lead them to action.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Crowded Communities - The all-in-one community platform.
Bylde - Bylde is a SaaS platform you can use to Start, Manage and Grow your groups and communities.
Ning - Ning is the social platform for the worlds interests and passions online.