Every few years or so there comes along a 'killer application' that catches everyone's fancy. For instance, we've seen the viral effect of Instant Messaging and SMS. While people like to communicate, many also love to listen to music. Remember your custom-made CD full of your favourite songs-especially those rare numbers no longer available in music shops. That's one factor driving the MP3 craze. It is actually the Internet and song-swapping services that have made MP3 another killer app. After downloading all those songs wouldn't you like it if you could also listen to them while travelling, or exercising. Let's take a look at hardware MP3 players and related trends.