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.
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.
Cattr - Cattr is a free open-source self-hosted time tracker with desktop clients for windows, mac, linux.
Trackabi - A robust and flexible online tool for time tracking and leave management for freelancers, consultants, small and medium-sized companies.
RescueTime - Time management software that shows you how you spend your time & provides tools to help you be more productive.