$9.99 (iTunes link
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.
Tons of features, left-hand mode, alternate tunings, and one of the only apps to offer a chromatic tuner.
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.