I purchased a BeBeSounds fetal doppler at Babies R Us, and I gotta say, for $20 it works AMAZINGLY. On the back of the pamphlet, it mentioned that the dopplers which Doctors use in the hospital produce an artificial heartbeat, where as this takes all the noises in the womb and just makes them VERY loud for you to hear. There is an adjustable volume control (I suggest you don't turn it up!), and two headphone jacks, so your boyfriend or husband (or loving Mother!) can listen with you. The coolest thing, when the baby kicks, the monitor picks it up! You can hear everything from YOUR heartbeat, the baby's heartbeat, and the bloodflow (whooshing noise) in the placenta. I wouldn't recommend you get this if you're under 20 weeks, since it takes me around 10 minutes to pick up a good reading of the baby's heart. They say it works best if you're in your third trimester.
Just thought I would review this for you girls, in case renting one online is not an option for you.