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.
Hourglass - Hourglass is the most advanced simple countdown timer for Windows.
Harvest - Simple time tracking, fast online invoicing, and powerful reporting software. Simplify employee timesheets and billing. Get started for free.
SnapTimer - SnapTimer is a simple, free, portable countdown timer for Windows.
RescueTime - Time management software that shows you how you spend your time & provides tools to help you be more productive.
Be Focused by XWaveSoft - Simple Pomodoro timer in your Mac's menu bar