A customizable interval timerUPDATE September 2016:
With Google phasing out the Chrome apps, this app has been moved to a progressive web app. It now runs offline, has updated sounds, and offers support for multiple timers. You can still add it to Chrome like a Chrome app but you can also add it to your Android, iOS, and Windows phones. See it in action here: https://interval-timer.wbrawner.com/
For support and updates, see here: https://github.com/wbrawner/interval-timer