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.

Cattr - Cattr is a free open-source self-hosted time tracker with desktop clients for windows, mac, linux.

Harvest - Simple time tracking, fast online invoicing, and powerful reporting software. Simplify employee timesheets and billing. Get started for free.

TopTracker - Beautiful, free time tracking

RescueTime - Time management software that shows you how you spend your time & provides tools to help you be more productive.

TimeCamp - Simple and robust time tracking app to help you stay on the same page with your team while working from home.