Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.

Todoist - Todoist is a to-do list that helps you get organized, at work and in life.

DownToMeet - DownToMeet is a platform to organize, find, and attend group events.

Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.

Mighty Networks - Mighty Networks enables entrepreneurs, organizations, and companies to create and grow a community-powered brand.

EssentialPIM - EssentialPIM is a free Personal Information Manager that keeps up with the times and lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.