Featuring one of our favorite interfaces on the list, studio.HD is an app that lets you record up to 24 tracks, then quickly edit and mix your song. Have an idea in your head that you're afraid to lose, studio.HD lets you record vocals using the built in mic. Editing is incredibly easy thanks to its familiar but tuned-up sound editor design. Also cool is the bevy of loops available to you. You can download any of the 900 free loops that span nearly all genres of music, cop some new ones from the in-app store, or import your own.