Tunebox for iOS

Tunebox is a neat iOS app that acts as an audio player for your Dropbox account. It automatically scans your entire Dropbox folder for any MP3 or MP4/AAC audio files, then organizes them the same way the built-in Music app does: by Artist, Album, and Song. You can save any track for offline playing, or just stream whatever you want as long as you've got a connection.

It's a $5 universal app that does a single thing very well. Check it out.