After I began learning guitar last year, I grabbed a copy of Agile Partners' guitar companion app, GuitarToolkit [App Store, $9.99] to help me learn chords and keep my guitar in tune. But after a while, I wanted a way to store tabs and music to look at while I was away from my computer. TabToolkit [App Store, $9.99], which was released in late September, is exactly what I needed. It allows you to store tablature and music notation in various forms and play them back in landscape or portrait mode. It even comes with a synthesizer that'll play the parts back to you and a metronome to keep you on time.
The bottom of the screen can display a fretboard (guitar or bass) or keyboard for finger positions while the top shows musical notation and chords. If you play guitar or piano, you should definitely check this app out!