Guitar Toolkit packs a ton of guitar tools into a single app. For $9.99, you get a chromatic tuner, chord finder, scales, four-string and five-string bass guitar modes, and a metronome. If you're going to buy just one guitar app, this is it.
Pros: Tons of features, left-hand mode, alternate tunings, and one of the only apps to offer a chromatic tuner.
Cons: If you can live without the tuner and bass modes, less expensive apps such as ChordMaster and ScaleWizard cover many of the same features. Tuner cannot be used with first-gen iPod Touch and requires a microphone accessory for use with second-gen Touch.
Strobe tuners are the holy grail of instrument tuning, offering accuracy within one tenth of a cent. Since a reliable standalone strobe tuner will set you back $75-$100, most guitarists make do with more affordable chromatic-style tuners.
Pros: Precise, professional tuning that can be applied to guitar, bass, brass, and woodwind instruments. Internal signal boost allows for direct input of guitar with a cable adapter. Noise filter helps to reduce affect of ambient noise on tuning accuracy. Concert A reference mode allows for piano tuning.
Cons: Requires a $13 instrument cable adapter for direct guitar input. Incompatible with first-generation iPod Touch, and second-gen Touch owners will need to purchase a microphone accessory or cable adapter.
FourTrack is a deceptively simple-looking multitrack recorder and mixing console. If you're a songwriter looking for a way to sketch multi-instrumental song ideas on the go, FourTrack acts as a fully baked portable recording studio.
Pros: Familiar interface for anyone who's worked with recording gear, real-time volume metering, track bouncing for limitless layering, and support for dock-connecting stereo microphones.
Cons: The app is relatively expensive, iPod Touch users will need to purchase a mic or headset, and recordings only transfer over Wi-Fi with no option to e-mail or upload to a Web host.
Like ChordMaster, ScaleWizard just tackles one aspect of the guitar (scales), but does a very thorough job.
Pros: Beautiful interface, tons of advanced parameters (arpeggios, modal scales, rare scales such as Semi Locrian b4). Landscape position offers full-screen fretboard view. Info screen includes options for left-handed orientation, alternate tunings, and automatic scale playback.
Cons: Not all-encompassing like Guitar Toolkit. Added background info or advice on scale choice would be helpful for novices.