If you need to monitor your heart rate during your workouts, 60beat is a neat little gadget that can help you. Its simply connects to your iPhone via the 3.5mm headphone jack and then is able to collect any signals being sent from a 5Khz or Polar compatible heart rate monitor chest strap.
The 60beat does come with it own basic monitoring application but data collected can also be shown in many of the exercise and running apps currently available in the iTunes app store.
Including: LogYourRun, LogYourRide, that are available for $2 each, both are then able to combine your heart rate with recorded GPS information and show critical positions on a map view of your run.
The 60beat Heart Rate RX monitor is available to buy for $29.99 and comes with a basic free iPhone monitoring app.