In response to Fil's "no" to a heart monitor, I somewhat disagree. While it's not 'essential', it is a great tool to improve your cardio workout.
I took a seminar at The Running Room, and a heart monitor was highly recommended, and helped my progress greatly. You burn calories differently depending on your heart rate, and thus, keeping your heart rate in the proper zone will improve this burn, and improve your overall cardio fitness.
Think about this. You start jogging 30 minutes a day, and it's fairly hard work. Without a monitor, you may never realize that you're going over 90% of your maximum healthy heart rate, and this could be dangerous.
On the latter side, you start jogging 30 minutes a day, and sweat and feel like you've done an effective workout. Without a monitor, you may never realize that your heart rate never went above 60%, so you're not getting the optimum level that you're training for.
Keeping your Heart Rate between 60%-70% is your Fat Burning Range.
Keeping your Heart Rate between 70%-80% is your Cardio Endurance Range.
Keeping your Heart Rate between 80%-90% is your Cardio Performance Range.
That being said, you need to know what range you're training in, and the only tool that will give you an accurate measurement is a Heart Rate Monitor. (Or, of course, you can stop your workout and try to get an accurate measurement from your pulse, but it's not as easy to be 100% accurate.)