Mobilizon - A free and federated tool to get our events off Facebook
Disciple - Grow, engage and monetise your community with Disciple community software. Get your own branded community platform on Web, iOS and Android
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Forumotion.com - Create a free forum : free forum hosting, free forum skins, templates editing, chatbox, RPG mod... Choose PhpBB 2 - 3, Invision or PunBB. Create forums now!
DownToMeet - DownToMeet is a platform to organize, find, and attend group events.
Proboards - ProBoards is a host of free forums with customer service.