Ubuntu One Music
As an Ubuntu fangirl, any way I can get my Droid to work with my computer running Ubuntu makes me happy, but this one works very well. Though you have to pay for a mobile account with Ubuntu One, if you're already backing up your media to the cloud, this should be no hardship. This app lets you stream your music library remotely from your phone, including letting you cache songs for offline play. A really great space-saver for your SD card.