BuddyPress - WordPress social networking in a box.
Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
Elgg - Opensource social network engine
Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.
SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.