Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
TaskPaper - Plain text to-do lists for Mac
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
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
Hero - Hero offers student behavior management applications for K-12 schools.