Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.
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.
TidyHQ - Organization management tool to support membership, subscriptions, event planning, and outreach.
Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.
Nabble - Nabble is a forum software that are embeddable and fully customizable with scripting language.