On a PC or Mac, Spotify is great. The app is easy to download and install, and the ad-supported free version of the online music streaming service works well enough that casual listeners don't need to shell out $5-$10 per month for Spotify's premium features. The company even recently added streaming radio, similar to what services such as Pandora offer, creating artificially intelligent radio stations on the fly from your favorite artists, songs, or playlists. Until now, the mobile app version of Spotify, for iPhones and the iPad, has only been available to premium subscribers at $10 per month. While there's still no free on-demand music for phones and tablets, a new version of the streaming radio service is now free to use on Spotify's iOS app (with Android support hopefully coming soon), giving non-subscribers their first real reason to download it. And that's exactly the game plan: Get the cheapskates (like ourselves) to download the mobile app, and give them a taste of Spotify on the go. Hopefully, after checking out the streaming radio, you'll want to access all the premium features as well and become a paid subscriber. Test-driving radio stations on the iPad, the system looks and feels very similar to Pandora, which has always had a free mobile version. There's the same thumbs up/thumbs down options for rating songs, and both Pandora and Spotify radio limit the number of times you can fast-forward to the next track to a handful per hour (because of music licensing issues). But, despite the similarities, the services actually operate quite differently. Pandora links songs together by tempo, mood, and other classifications, relying on a huge cache of descriptive data about each track, called the Music Genome Project. Spotify, on the other hand, uses social sharing to determine what it thinks you want to hear. Start with a particular track, and Spotify will check to see what other people who listened to the same song also "liked," either in the app or on Facebook. The practical effect is a collection of related music that's less precisely tailored, but may also be more adventurous, than what Pandora offers. In short, you'll get a lot more left-field choices with Spotify's radio service. For now, Pandora is still the king of streaming internet radio, with 150 million registered users, compared to about 10 million for Spotify. Fortunately, both iOS apps (and their PC/Mac versions) are free to download and use, so it's easy to test drive them both and see which you like better.