So shoot me, I nabbed the last one for 99 cents in a hoarder's panic: http://www.ebay.com/itm/5V-Heartbeat-Sensor-Senser-Detector-Module-By-Finger-For-Arduino/191129736978
Looks to be nothing more than an IR LED and phototransistor. Just like a line follower sensor. How well can this work? Has anyone tried one like this? Must be some signal conditioning required.