Baby Monitors, Sleep Monitors, Cot Death Alarms
I think you can split Baby Monitors into two Categories
There are those worn by the baby or in direct contact with the baby or there are baby monitors which alert you to the surroundings of the baby and the room they are in.
Contact Baby Monitors
The Baby Monitor that kept my wife and I sane through the first year of our son's life was the Respisense Buzz Monitor as shown below.

Respisense Buzz Baby Monitor
This little Monitor fits on to the baby's nappy and if it detects no movement makes a buzzing noise, if it continues to detect no movement then it sends out a very loud alarm noise. We had very few examples were it sent out false readings and this mainly happened when he weed so much at night that the nappy expanded and the Monitor lost contact with his belly.
When the alarm goes off you know about it as I used to leap out of bed. But the important thing we had been able to get to sleep in the first place. Some of the health professionals were a bit surprised that we would have a sleep Apnoea Monitor but if you are trying to avoid Cot Death then it is a small inconvenience and relatively little cost compared to potential trauma.
As with many of these things it is all about your own perception of risk and mitigating against it. In our view there was a risk of Cot Death and this Monitor helped mitigate against it and allowed us to get some much needed sleep. It can be a bit fiddly with the on button and you have to baby clothes on top of them as well. Highly recommend.
There is a newer version now on the market, which has slightly more features and the battery is now replacable.

Latest Respisense Design
As well as direct skin contact monitors there are baby monitors which they lie on. My wife and I were less sure about these because most seemed to be powered and hence there was a trailing cord and we were as worried about that. Friends who tried them said that they were okay but fiddly and I think there were more false alarms than what we got with the Respisense Alarm. We also had a lot of vomit problems early on and so weren't too sure about how they would stand up.
Nanny Baby Breathing Monitor
Baby Monitors
The main other type of Monitor records the room that the baby is in rather than the Baby itself. So you can hear noises, find out what the temperature is etc and most importantly do this remotely. The commonest Baby Monitors have a Baby Unit and a Parent Unit. With all the radio signals, mobile phones and wifi in a modern house it can be hard to pick up a completely clear signal.
One word of caution is to check what is meant by digital. It often just means the display. Check that the Parent Unit is battery operated as well as mains. With the Temperature we found that it did record a 1C degree higher than the room with our TOMY Monitor after it had been in operation for over an hour. I think the Baby Unit warmed up and influenced the temperature sensor.
There are lots of extra features on these, sound lights, night lights, talk back etc and it just depends upon what you regard as essential qualities for the Baby monitor. The more features the higher the price.
As well as traditional manufacturers such as Tomy, Philips AVENT and Summer the phone manufacturers have also moved into this market. So Motorola, Binatone and BT also have Baby Monitors on offer. It depends upon how many times you feel you might have to zoom in and look at little Johnnie over a Monitor rather than just listening to hear if he is okay.

Motorola MBP30 Baby Monitor
