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.
TopTracker - Beautiful, free time tracking
Paymo - Manage projects, track time, create invoices, and get paid online.
RescueTime - Time management software that shows you how you spend your time & provides tools to help you be more productive.
ManicTime - Track your computer usage and use collected data to accurately tag time.
Cattr - Cattr is a free open-source self-hosted time tracker with desktop clients for windows, mac, linux.