We use (Still) the babysense II breathing monitor. It comes with a sound monitor. We've had a couple of false alarms with it, and when we moved onto floorboards we checked if the computer etc would simulate breathing. The puters did, so we placed the cot on an old mousepad (neoprene style) we cut up into 4.

It gives us peace of mind, but if Pip's having a restless night I still hold him close to me.