BuyDig offers the Garmin Heart Rate Monitor, model no. 010-10997-00, for $14.99 with free shipping. That's $1 under our mention from three weeks ago and the lowest total price we've seen. (It's the best deal now by $7; most stores charge beyond $25.) It wirelessly transmits and records information about your heart rate.
Note that this is just the monitor and a compatible device is required to make use of its features.