I recently purchased this great little tool from the US to help me when I do my High Intensity Interval workouts. It's basically a timer that you can program to beep at set intevals which allows you to work out for a set period of time and rest for a set period of time. I bought the basic model that allows you to set 2 times. If you like to do circuit training then I would recommend you get this gadget if you don't already own a timer of some sort.
http://www.gymboss.com/?src=IAP
