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.
ManicTime - Track your computer usage and use collected data to accurately tag time.
RescueTime - Time management software that shows you how you spend your time & provides tools to help you be more productive.
Qbserve - Freelance time tracking, invoicing, and productivity improvement
Kimai - Kimai is a free open source timetracker.
Harvest - Simple time tracking, fast online invoicing, and powerful reporting software. Simplify employee timesheets and billing. Get started for free.