You will be amazed at how many local radio stations, both AM and FM, are broadcasting in HD. Many stations plan to broadcast in HD soon, too. Why? The sounds you hear are crystal clear and do not fade in and out like analog signals. You may listen to a simulation of HD and non-HD radio on the hdradio.com site.
To listen to digital AM and FM programming, all you need is an HD radio receiver. The beauty of HD radio is that the digital signal is piggybacked on the analog signal, so there is no cost for you to listen. The digital part of the signal contains information that is not sent with the analog signal, like information about the song title and artist.
This extra bit of information sent with the digital signal enables a neat feature called song tagging. When you hear a song on the FM band of the HD radio that you like, just hit the "tag" button on the radio. The song and the song information are saved to your iPod. Your selected songs show up in an iTunes play list as "tagged." This allows you to click, download and buy the songs you want directly from the Apple® iTunes Music Store.
Many new cars come with HD radio receivers. To add an HD radio receiver to your car ranges in price from $100 to $400. If you want to listen to HD radio at home, you will need a Jensen JiMS 525i (pictured below) or other name brand HD radio receiver. Portable HD radio receivers start at $100.
I expect all radio stations will be broadcasting in digital eventually. If you wish to add HD digital radio to your home entertainment center or home theater, you may spend from $200 to $7000, depending on the brand and the features you want.