I am in love with the Nike + iPod Sports Kit. It comes with a receiver that attaches to your iPod nano, and a sensor that fits into your Nike sneakers. It allows you to keep track of the time, distance and calories burned during each of your workouts. You can even send your data to NikePlus.com, where it stores all your workouts and allows you to set personal goals. The only bad thing is you have to use the Nike Sneaks that are specially designed to hold the little sensor.
What if you don't like Nike sneakers because you've always been a New Balance girl? No problem. You can still use the Nike + iPod Sports Kit with your favorite sneakers. You just need to purchase a separate holder that attaches to your shoe.
I just found this one called the Shoe Pouch ($9.95). It's made of neoprene and has little loops that you slip your shoelaces through, so it fits snug against your shoe and stays put. The sensor fits inside the slit in the back of the pouch, and even has a Velcro closure to keep it extra secure.
Fit's Tips: If you run and walk in the rain or in wet areas, you may want to consider getting the RunAway ($7.99) instead. Although it may be waterproof, it only slips over your laces (not through them), so it doesn't fit nearly as securely as the Shoe Pouch.