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.
Microsoft To-Do - Task management tool
TickTick - TickTickis a cross-platform to-do list app & task manager helps you to get all things done and make life well organized.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Zenkit - Zenkit is the home you always wanted for your ideas, your projects, your business.
doo - A smart, simple app for reminders and to-dos