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.
NetTime - nettime is a time & attendance software solution.
Harvest - Simple time tracking, fast online invoicing, and powerful reporting software. Simplify employee timesheets and billing. Get started for free.
Dimension 4 - The fastest and easiest way to synchronize your Windows clock.
RescueTime - Time management software that shows you how you spend your time & provides tools to help you be more productive.
Meinberg NTP - NTP client and server download for different operating systems.