How often have you listened to music in the office, and not realized someone's shouting at you? Or unknowingly missed an announcement on the train that the course was changing? "Awareness! The Headphone App!" could be a $5 life-saver.
Available for both the iPhone and iPod Touch, it utilizes the inbuilt microphone and feeds in relevant outside noise over the music. How does the app know what's relevant or not? Well, it's probably not accurate 100 per cent of the time, but any sharp noises (such as a car horn, or someone suddenly shouting) will make it over your David Bowie, but constant noise, such as a jackhammer or people talking next to you, will remain blocked out.