Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
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.
Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.
Microsoft To-Do - Task management tool
Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.