Things - Things is an easy to use task manager.

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

BallerTODO - Ditch the list! Organize like a baller, get stuff done.

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

Todoist - Todoist is a to-do list that helps you get organized, at work and in life.

Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.