Interval training is popular among those who look for increased performance. The concept can be summarized as: short periods of high intensity balanced with periods of lower intensity or even rest. These periods of high intensity can be for running (sprints), high repetition weight training, kettlebelling, or gymnastics movements (pushups, situps, squats).
One of the niftiest gadgets on the market is this interval timer. The GymBoss allows the user to program two separate (or just one) countdown timers....one for rest period and one for burst period.
Interval timers have been recently integrated into heartrate monitors also. But if your HR monitor does not have a timer, or if you're not interested in monitoring your HR during workout sessions, this $20 unit is for you. Goto http://www.gymboss.com/ to purchase online. You won't be disappointed.