Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
HeySpace - HeySpace is the only free task and simple project management software with communication and collaboration feature.
Any.DO - The #1 task management app used by over 11 million people globally, Any.do is your free mobile and online task manager for Android, iPhone, Web and more.
Rivers IM - Simple, fast & secure team collaboration platform
Microsoft To-Do - Task management tool
P2 - Share, discuss, and collab with your team. By Wordpress.