Gone - An ephemeral to-do list
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
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.
Check - Check is a unit testing framework for C, which runs on environments such as GNU/Linux, GNU/Hurd, BSD, and Mac OSX.
Microsoft To-Do - Task management tool
Things - Things is an easy to use task manager.