Doit.im - Doit.

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.

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.

TickTick - TickTickis a cross-platform to-do list app & task manager helps you to get all things done and make life well organized.

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.