If you want to keep your music synced with iTunes, there are several nice options.
doubleTwist is the best in my opinion. It syncs with iTunes, has a nice user interface, and is free.
I use
Google Play Music, which uploads all your iTunes music to the cloud and keeps it synced (it will not sync new metadata however; only new/deleted songs.) This keeps your internal storage free for other stuff. If you have an unlimited 4G connection this is great, but if you frequently in areas without network coverage then choose a different option.
iSyncr ($2.99) and
TuneSync ($5.99) work great as well, but they aren't free.