How the mp3 player took over the world and changed how we travel.

The sign outside the small Costa Rican guesthouse read, "Rooms for Travelers. Good and Cheap!" And I spent the first night in my good, cheap room listening to my mp3 player, a Zen Nomad, on my fist-size Creative travel speakers. The next day, the guesthouse proprietor, a tall and thin Caribe man with gray hair and a bluish face, approached me. "I heard you listening to those old blues that I like," he said. "Do you have "Stand By Me?" I did. Actually, to his amazement, I had five versions of it, and we listened to them all.

My experience in Costa Rica, however, is far from unique. Even before Apple released its world-dominating iPod in 2001, high-tech travelers were stuffing their Rios and Nomads with everything from Bach to blues to Britney. Today, though, the mp3 player has become more than just a mega-Walkman for wowing tech-deprived locals: It's a library, a language school (Fodor's "Languages for Travelers" series), an atlas, a photo album, a field studio (when that flute player at Angkor Wat isn't selling CDs, you can always record him), and more. And tomorrow, it'll be something else again.

Businesses are beginning to catch on to the mp3's ubiquity. Hostels like the Bearded Monkey in Grenada, Nicaragua, have realized that everyone has one, and let you download albums and individual songs directly to your player (for a fee, of course). Audio book publishers like Penton Overseas and Soundwalk (and even Virgin Atlantic!) are releasing walking tours of many European and American cities, as well as travelogues and guides to particular topics, like wine and history. Adventure Guides and Let's Go are now offering their books in PDF format, so that you can print just the pages you need from any cybercafe, as you need them, rather than lugging the six-pound tomes across Mongolia. (Soon you won't even need to print them - mp3 makers are adding PDF-display capabilities to their players.) And in New York, the folks behind the pizza blog Slice have released piPod, a neighborhood-by-neighborhood guide to the city's best pies.

But it's not just corporations, big and small, that are boosting the functionality of your mp3 player. Travelers around the world are creating and releasing podcasts of their adventures - essentially radio-style narratives in the mp3 format. One describes a New Yorker's trip to India to learn Ashtanga yoga (mekambo.com/podcast/yogatravelog.xml). In "Dead Pirates Society," three Utah college students ditch school to explore the Caribbean (deadpiratessociety.com). And a trucker named Tom creates "Tom's Trucker Travels" right in his rig (truckerphoto.blogspot.com). A few others - Red Eye Radio, The Road Less Traveled, and Travelcommons - discuss an array of travel advice and topics for everyone from the budget traveler to the on-the-road executive.

Making your own podcast is simple. Simply record whatever you want, edit it with free software, such as Audacity 1.2.3., then just upload it to your own Website, iTunes, ourmedia.org, audiblog.com, blogger.com, libsyn.com, or any of the other dedicated podcast sites. If you want it spelled out for you, you can just go to iTunes, click on "Submitting My Podcasts," and follow the step-by-step instructions.

Still, the power of the mp3 player continues to reside in its vast capacity for storing music. Recently, on a riverboat in the Amazon, I explained to a local man exactly what it was that I was listening to. He couldn't believe that the mp3 player, this tiny machine, held what seemed to be every song ever written. He almost cried.

ESSENTIAL EXTRAS
An mp3 player just isn't complete without a few high-tech add-ons. Here's what you need to get the most from your magical little box.

• FM radio: For some reason, this basic feature gets left out of most CD players and walkmans. But some mp3 players from Sony, iAudio, and Creative allow you tune into AM, FM, and shortwave broadcasts no matter where you end up.
• Audio recorder: Whether you're collecting material for your podcast or just want to remember the musical stalactites in the limestone caves of southern Cambodia, you should bring along a Griffin iTalk Voice Recorder ($27).
• FM transmitter: Okay, so your iPod has every Rolling Stones tune ever recorded. Share them with the rest of the tour bus by plugging in a Belkin Tunecast II FM Transmitter ($29.99) transmitter and blasting "Sympathy for the Devil" all the way from Rio to Buenos Aires.
• Camera cable: This lets an iPod download photos directly from your camera. Try the Belkin Digital Camera Link for iPod ($29.95).
• Speakers: The Creative TravelSound 200 Portable Speaker System ($34.99) fits in your palm but rocks like an arena amp.