doo - A smart, simple app for reminders and to-dos
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Purp - A smart to-do list and goal tracker app
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.
Gone - An ephemeral to-do list
Zenkit - Zenkit is the home you always wanted for your ideas, your projects, your business.