Audioshare (MUST HAVE music app)-> Free
I bought this for 5 bucks, IIRC, and have never regretted it. This app makes it easy to store sounds/tracks for inter-app use, and, more importantly, is probably the easiest solution for uploading music to soundcloud/dropbox, etc.
If you ever want to get self-made music off of your device, this app is an essential tool. And it has never been free before.