A heart rate monitor can accurately measure your work-out, allowing you to be more efficient (and safe) during all kinds of activity and training.  Personally, I use the Polar Beat...  one of the less expensive models.  It has fewer bells and whistles, but has everything I need in a heart rate monitor.   My fitness training is far more effective with my heart rate monitor.  Personally, I've owned only Polar heart rate monitors and they carry a wide selection depending on your needs.

A wide range of monitors are available...  from around $50, up to $200.  To check out a  variety of heart rate monitors, including their features and prices, click here