Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
Taskade - Make lists, organize your thoughts, and be inspired to get things done. Taskade is a collaborative space for your tasks.
Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.
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.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.