In the school at which I teach we have eighty minute class periods. Eighty minutes is a long time for anyone, let alone teenagers, to stay in one room and on task. Therefore, I break up the time with varied activities and "break times" during the eighty minutes. One tool that can help to get the students and me to not stretch the "break times" is to use a countdown timer.
Online Stopwatch is a free website that gives you the choice of a stopwatch function or a countdown function. You can set the countdown timer for any length of time and an alarm sounds when time is up.
Online Egg Timer is a simple website offering three countdown timers on one screen. You can set just one timer or run all three at the same time with different settings. No registration is required in order to use Online Egg Timer. Just go to the site, set the countdown timer(s) using the up and down arrows, then click "start timers."
Timer Tab is a free application offered through the Chrome Web Store and as a stand-alone website at timer-tab.com. As a Chrome app Timer Tab can be used online or offline. The website version of Timer Tab can only be used online. Both versions of Timer Tab offer a count-down timer, an alarm clock, and a stopwatch. For the count-down timer and the alarm clock you can select a video to play when time expires or when you've reached your specified alarm time. You do this by pasting the url of a YouTube video into the "alarm" field below the timer. Also in both versions of Timer Tab you can customize the background by specifying an image url.
Timerrr.com offers two versions of their free countdown service. The regular Timerrr displays a dial like my grandmother used to have on top of her stove. The "egg timer" version displays an egg timer as the countdown device. Both can be used to set a countdown of up to sixty minutes.