Harvest - Simple time tracking, fast online invoicing, and powerful reporting software. Simplify employee timesheets and billing. Get started for free.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Toggl - Toggl is an online time tracking tool. It features 1-click time tracking and helps you see where your time goes. Free and paid versions are available.
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.
RescueTime - Time management software that shows you how you spend your time & provides tools to help you be more productive.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.