On to heart rate monitors... with every workout, I'll be wearing my HRM so I can track my calories. It helps me figure out how much to eat. I have a Polar F11, but the Polar F6 does the same thing mine does and is cheaper. I highly recommend it! You enter your age, weight, sex, and fitness level. Then it estimates your VO2 Max based on your resting heart rate and how it fluctuates over a few minutes. When you exercise, it measures your heart rate and using all the other information to calculate calories. I think it's pretty accurate... definitely more accurate than the treadmill at the gym. I've heard from some hardcore calorie counters that they think it overestimates because it also takes into account the calories you would've burned just sitting around. IMHO, that's not that many calories (maybe 100-200?) and it's not going to make or break me.
In case you want to buy one: