There are loads of timer applications available which offer a range of flexible functions. Lightning Timer does just one thing: counting down from 5 minutes to zero.
As the name suggests, it's designed for timing "lightning talks" at conferences and seminars, which typically have a 5 minute limit. Visit the site and click Start and the countdown begins, in numbers so large that they can be seen from some distance. Once the timer dives under one minute, the screen starts flashing. It's a dead simple and highly specific application, but really useful if you want a quick timer that's visible to lots of people for your next conference (or unconference) .