I'm super cheap. My 911 has three cameras in it, now. They're $38 each, and I added a $15 wide angle lens to each one (although they now have a $44 model with a 120° wide angle lens). They record in true 720p HD, with audio, and are constantly charged through a USB cable. They record to the small SD chips, and can be set to record in continuous a loop.
Setting them up requires a little more work than some of the cameras mentioned here -- but like I said, I'm cheap.
You can see the rear-facing one on a little bracket I put on my license plate.
Here's a clip shot by that camera.
The camera is an 808 #16 key fob camera. You can learn more about it
here.