RescueTime - Time management software that shows you how you spend your time & provides tools to help you be more productive.
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.
Clockify - Simple and free time tracker. Perfect for small and mid-sized businesses as well as freelancers. Unlimited projects and users, unlimited productivity. Get all the premium functionalities, completely free.
Harvest - Simple time tracking, fast online invoicing, and powerful reporting software. Simplify employee timesheets and billing. Get started for free.
Kimai - Kimai is a free open source timetracker.
Qbserve - Freelance time tracking, invoicing, and productivity improvement