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.