BuyDig offers the Garmin Heart Rate Monitor, model no. 010-10997-00, for $23.99. Coupon code "HRMDEAL" cuts it to $17.99. With free shipping, that's tied with last week's mention and the lowest total price we could find by $4. 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.