Morning, I have been toying with buying a Doppler but have just managed to hold back, THEN I found an app on my iPhone which uses you phone as a stethoscope!!! 
It's called Tiny Beats and it was only £2.99!
You put your phone on aeroplane mode, put earphones in your ear then position the microphone on the bottom of the phone on various places on your tummy and eventually (after getting nervous) it will pick up your baby's heartbeat and show you a graph of it on the screen and the beats per minute (mine was 145bpm).
Best to find your own heart first so you know the difference.
You can also save and email recordings!
I have to say it wasn't very clear to actually hear, I'm sure a Doppler is far better at this stage (app says best from 30 weeks) but as it identifies the heartbeat and bmp for you, you know when you find it.
Also, because it's technically to early to be able to hear it, I think it being hard to locate panicked me much less than if it was a Doppler!