Amazon MP3 with Amazon Cloud Player for BlackBerry PlayBook
The Amazon MP3 for Android app is designed for use on handhelds, so the software's UI isn't exactly stunning on the PlayBook. But it does give you online access to any and all music you may have stored in your Amazon Cloud Drive, and it's easier than firing up a browser to access your music every time you want it. I personally have upwards of 20GB of tunes on my Cloud Drive, so this app gives me access to a significant chuck of my digital music library, and I don't have to waste any valuable storage space on my PlayBook. The Android app lags a bit on the PlayBook, but it works fine.