Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
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.
Asana - Asana project management is an effort to re-imagine how we work together, through modern productivity software. Fast and versatile, Asana helps individuals and groups get more done.
Google Tasks - to-do list app that integrates with Gmail and Google Calendar
WorkFlowy - WorkFlowy is an organizational tool that makes life easier. It's a surprisingly powerful way to take notes, make lists, collaborate, brainstorm, plan and generally organize your brain.
Things - Things is an easy to use task manager.