The Fetal Doppler uses ultrasound technology to bounce sound waves off the baby and return a representation of the fetal heart beat. Some specialized devices can be used as early as eight weeks. Though 12 weeks into pregnancy is a more normal time frame. The sound is usually that of galloping horses. (You can hear a baby's heart beat via doppler here.)
This device can be used by doctors or midwives. There are companies who sell or rent them in pregnancy for home use. This is not an approved use according to the FDA.