I love the Slacker and Pandora websites — free services that let you create your own radio stations that only stream music that you like. Both are very popular, and both have iPhone apps. Now Slacker has taken an interesting — and timely — new direction. Given the constraint on data usage in AT&T's new plans, the ability of Slacker Radio 2.0 (released today) to cache songs can be a convenient and money-saving feature. You can simply tap Cache This Station when you're listening to Slacker's streaming music via the iPhone app, and then the songs will be stored for later listening when you're offline. For example, if you have Wi-Fi at home, you could cache hours of music and then listening to it while you're out and about — without incurring data charges. Or you could listen at times when no service is available, such as on a plane. While Slacker is free — both the app and the service — you need to be a Slacker Plus subsriber (as low as $3.99 per month). Once the songs are cached, those that you mark with a heart are retained so that they can be used in the future as Slacker structures your playlists. That means that future caching will be much faster.
[post updated with additional information 6/4/10]