Agile Partners (who've come up at TUAW a few times) have released GuitarToolkit 2.0, a major upgrade to the original app which adds extensive features and an iPad version. GuitarToolkit boasts a range of tools geared at guitar and bass players, but now includes enough flexibility to work with any fretted instrument you can imagine. This is a free update for existing customers, and additional functionality is available as a one-time, US $4.99 in-app upgrade.
In addition to being redesigned to take advantage of the iPad's size, the new release expands from 500,000 chords to over 2,000,000 chords and scales--and now--arpeggios. There's full capo support, too: slide to any fret and scales and arpeggios automatically adjust. All of GuitarToolkit's features support six, seven, and twelve string guitars, as well as four, five and six string bass, banjo, mandolin and ukulele.
The GuitarToolkit+ upgrade activates interactive Chord Sheets (a great tool for composition and building chord progressions), an Advanced Metronome (with visible flash option), a drum machine and Custom Instruments. Drum patterns can contain as many as 32 tracks, and you control time signature, number of bars and have access to 75 sounds in nine different categories. The Advanced Metronome is available immediately on the iPad, and coming shortly to the iPhone/iPod touch.
The Custom Instruments capability is pretty cool, especially if you're playing something that doesn't necessarily fit into the standard guitar mold. Choose an instrument type, string and pickup type, assign a tuning, even add a capo if you like. GuitarToolkit+ includes great-sounding samples for every combination, as well as over 60 amp/effects presets powered by AmpKit.
The tuner in GuitarToolkit is highly accurate and supports just about any tuning you can imagine (including custom tunings). More than I'll ever use, I'm sure. There's also a high-contrast mode to make sure you can use it in any lighting situation.
All told, if you play a fretted instrument of any kind, the US $9.99 app (and the US $4.99 in-app upgrade) will probably pay off pretty quickly. This is the only app I've seen that offers this kind of flexibility and provides as many composition tools. Check it out in the App Store.