Police scanners are radio frequencies by which officers communicate with each other while on the beat. You probably have heard them when you walked by a police car. Not only is it possible and legal for civilians to listen in on police broadcasts but there are plenty of live feeds on the Internet where the scanner of your local department is no more than a mouse click away.
Police scanner resources
While there is no official database of police scanner feeds in the United States (or internationally), Police-Scanner.info has links to plenty of relevant websites. Not only will they direct you to hundreds of police audio feeds but the website has a library of dramatic recordings, books & guides on scanning and a comprehensive history of the technology. If you just want to find a quick list of police scanner feeds in major cities, try the Amateur Radio Resource Group (ARRG) website. (See References).
Police scanner codes
Listening to your local police scanner may not be very informative until you understand the codes which they use to avoid elaborate on-air explanations. As with CB radios, "10-4" means the last message is understood. Other codes are less obvious. For example, in Waterloo, Canada, a "935" means an individual who appears to be intoxicated, a "967" means a search warrant and a "946" means a prowler. These codes range from municipality to municipality, so consult with your local department about their list of codes. They often keep a list of these codes online.
Scanner radio clubs
Though you might think listening to police radios to be a rather esoteric pursuit, there are plenty of meet-up groups and online forums for scanner enthusiasts. Though there is no central directory of these websites. For example, if you live in the Midwest, Chicago Area Radio Monitoring Association (CARMA) has more than 1,400 members with monthly meetings, e-mail newsletters and other resources.
It is completely legal to listen to online police scanners in the United States as long as they are not encrypted or scrambled. So if you want to listen in on an encrypted broadcast of an FBI counterterrorism operation in progress, consider another hobby. It is also illegal to use information you hear over a police scanner to commit a crime.
Reasons for listening
If you are a journalist who is interested in getting a "scoop" for a breaking news story, the scanner of your local police department can be very useful. For amateurs, police scanners offer the thrill of hearing about news stories in progress before they end up on your local TV station. Hostage dramas, bank robberies and other dramatic incidents are all covered in real time. In addition, civil libertarians can monitor police scanners if they suspect their local department is involved in professional misconduct.