From Simply Good Code:
A free online stopwatch and countdown timer. A large display that automatically adjusts to the size of your screen which makes it great for classrooms and large audiences. It can run in the background so you don't have to worry about starting over if you accidentally close the window. That feature also makes it perfect for measuring long running events, even ones that span days. Large buttons and a straight forward design make it easy to use on most devices including tablets and smart phones including the iPad.