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

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

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

Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.

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

CherryTree - A hierarchical note taking application, featuring rich text and syntax highlighting, storing data in a single xml or sqlite file.