I bought this AngelCare monitor when my first child was a small infant. It was the best $70 I'd spent and because of this monitor I could finally get a few hours of sleep.
This is not your traditional baby monitor, but rather a monitor that detects the tiniest movements of your infant breathing. A pad is placed under the crib mattress, and a cord attaches it to the angel shaped monitor that sits on a shelf or nearby table. As long as your infant is breathing, a green light flickers. If no motion is detected for a period of 15 or 20 seconds (which is longer than it seems!) a soft beeping occurs and the light turns to red. With each of my children, I heard the dreaded beeping a couple of times, but it was just because they'd rolled too far away from the sensor pad. The beeping was loud enough to wake me, probably because I'd told myself to listen for it, but it never once woke up the children.
The only thing I did not like was that the monitor was connected to the pad with a cord, which seems to go against everything we've learned about keeping cords away from babies. It stays on the outside of the crib, but it always did bother me a bit. (None of my kids ever got ahold of it, though.)
I recommend this item highly to any of my friends who are having babies, and I've purchased several to give as shower gifts. Giving the gift of sleep to a new mom is priceless!