I haven't actually had the time to buy/use the Awareness! The Headphone App ($4.99) yet, but the idea is sound. The Awareness iPhone/iPod Touch App basically lets you listen to your music and ports in external sounds that exceed the level of regular background noise. For example a semi-truck about to run you over or an a-hole in a bmw honking his horn.
Once you calibrate what "normal" noise levels are in your area this gadget could save your life?