Intervals is the easiest to use and most flexible interval, circuit training, crossfit and all around exercise timer available for the iPhone and iPod Touch.
Create as many workouts as you want. Customize each workout by creating a set of repeated steps. Steps can be time based or based on a number of repetitions. Add in warm up and cool down steps for a complete workout. A clear tone will alert you when each step is complete.
Share your workouts with friends on Facebook, Twitter, or by email.
Work out to music by starting your music first, and then launching Intervals and starting your workout. The music volume will be briefly lowered to ensure that you can hear the tone at the end of each interval step. Change songs or take a phone call mid-workout and Intervals will keep your timer running in the background (tone will not sound in background, but the correct time will be kept).