I have got the BT Baby Monitor Pacifier (http://www.shop.bt.com/products/bt-baby-monitor-pacifier-7HGG.html?src=2) which does a light show and plays various lullabies which is great for using at home. However, its not really ideal for travelling.
Its baby unit is pretty big and heavy and, more importantly, there is very poor to no reception as soon as there are a couple of thick walls between the hotel room where the baby is sleeping (or not...) and the restaurant where the parents would like to dine in peace. At a hotel in Switzerland, they had their own baby monitor system where the baby's crying triggered a call to the reception. However, I have not come across this since. I am therefore very excited by the "Baby Monitor" i-phone app a good friend of mine told me about. Once downloaded (there are different baby monitor apps so make sure you get the right one which has a white baby monitor on light blue background as its symbol), you simply have to place your i-phone in the baby's room and switch on the "phone alert". If the baby cries, the i-phone calls a pre-selected number. I have tried it and it works (I cried into my phone and it called my husband's number...;-))! Other features of the app are voice recording (you can record your own voice or the baby's favourite song which will be replayed when the baby cries and hopefully send it back to sleep) and a "sweet dreams" function which you can use to play your baby's favourite song or fairytale to help it fall asleep.