There's an almost-limitless number of reasons to use a countdown timer, and making your own is quite simple with an Arduino and some spare time. A neat example of this has been documented by Randy Sarafan, who describes how to make a complete unit that uses some classic 7-segment numeric LED displays for the timer. Once completed you could use it for photography, cooking, quiz night games and more.
The project is easily expandable to add more digits, and you could also modify the design to trigger various outputs to control buzzers and so on over periodic intervals. So to learn more, visit Randy's project page. And for more, we're on twitter and Google+, so follow us for news and product updates as well.
If you're interested in adding more than a few digits to your Arduino-based projects - save time and move forward with the Freetronics LCD & Keypad shield which contains a bright 16x2 character LCD and five buttons that can be read from only one analogue input pin: