Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
DownToMeet - DownToMeet is a platform to organize, find, and attend group events.
Nifty - Manage projects, work, and communications in one place.
Mighty Networks - Mighty Networks enables entrepreneurs, organizations, and companies to create and grow a community-powered brand.
Asana - Asana project management is an effort to re-imagine how we work together, through modern productivity software. Fast and versatile, Asana helps individuals and groups get more done.