When I turned 13 years old, I got a CB (Citizen's Band) radio for my birthday. It was my first hands-on experience with radio communications (other than toy "walkie-talkies"), and since then I have been a radio fanatic. My radio hobbies have evolved over the years from CB radio to scanning/monitoring (I currently own a Uniden BCD-396XT) and later into amateur radio and shortwave monitoring. You can imagine my excitement when I learned about the iPhone/iPod Touch app Emergency Radio by EdgeRift, which allows you to tune in to live radio scanner feeds (police, fire and EMS) from all over the country. Emergency Radio pulls the live streams from ScanAmerica (Now part of RadioReference). While you can already listen to the feeds on RadioReference, having a slick and portable interface on the iPhone/iPod Touch is very handy and can be very useful especially if you are visiting a city and you want to find out what's going on.
The free version, Emergency Radio Lite, lets you listen to Chicago, Denver, Miami, New Orleans, and San Jose, while the full version provides access to over 1,000 frequencies. There are a handful of radio scanner apps available, but Emergency Radio has the highest rating (3.5/5 as of today). (Sources: Emergency Radio, Macworld)