This website provides a large Simple Timer that can be displayed on your computer/projector. You can have the timer simply run, or set it to count down from a specific number of minutes. Simply choose the option you prefer, then click Start the Clock.
The clock will turn green when it begins counting, and will sound a gong when time has expired (if you have set the clock to count down).
Use this tool anytime you are in need of a timer in your class. Just bookmark the site on your computer so that it is easy and fast to access! If you like the Simple Timer you may also want to check out the One-Minute Timer.